feat(tools): add run_shell, edit_file, write_file + read_file directory listing

Four standard-tier tool extensions (one tool per extension, zero ABI change):
- tool-read-file: read_file now lists directory contents (sorted, /-suffixed subdirs)
- tool-shell: run_shell (foreground, streamed, cancellable, cwd, timeout + output cap)
- tool-edit-file: edit_file (oldString/newString/replaceAll; errors on absent/non-unique)
- tool-write-file: write_file (explicit overwrite flag)
Registered in host-bin CORE_EXTENSIONS. Live boot clean (shell capability accepted).
686 vitest + 89 bun = 775 tests; tsc -b EXIT 0; biome clean.

1 files changed, 210 insertions, 0 deletions

diff --git a/packages/tool-write-file/src/write-file.ts b/packages/tool-write-file/src/write-file.ts
new file mode 100644
index 0000000..16cfc83
--- /dev/null
+++ b/packages/tool-write-file/src/write-file.ts
@@ -0,0 +1,210 @@
+import { access, lstat, readlink, realpath, stat, writeFile } from "node:fs/promises";
+import { dirname, resolve, sep } from "node:path";
+import type { ToolContract, ToolResult } from "@dispatch/kernel";
+
+interface ValidatedArgs {
+	readonly path: string;
+	readonly content: string;
+	readonly overwrite: boolean;
+}
+
+export type OverwriteDecision = "create" | "overwrite" | { readonly error: string };
+
+/** Pure: decide the action based on file existence and the overwrite flag. */
+export function decideOverwrite(fileExists: boolean, overwrite: boolean): OverwriteDecision {
+	if (!fileExists && !overwrite) return "create";
+	if (fileExists && !overwrite) {
+		return { error: "Error: File already exists; set overwrite: true to replace it." };
+	}
+	if (fileExists && overwrite) return "overwrite";
+	return { error: "Error: overwrite: true but the file does not exist." };
+}
+
+/** Pure: check that a resolved absolute path is within the workdir (prefix check). */
+export function isPathWithinWorkdir(resolvedPath: string, workdir: string): boolean {
+	const normalizedWorkdir = workdir.endsWith(sep) ? workdir : workdir + sep;
+	return resolvedPath === workdir || resolvedPath.startsWith(normalizedWorkdir);
+}
+
+/** Pure: validate and coerce args from the model. */
+export function validateArgs(args: unknown): ValidatedArgs | { readonly error: string } {
+	if (args === null || args === undefined || typeof args !== "object") {
+		return { error: "Error: Arguments must be an object." };
+	}
+	const obj = args as Record<string, unknown>;
+
+	const rawPath = obj.path;
+	if (typeof rawPath !== "string" || rawPath.length === 0) {
+		return { error: 'Error: Missing or invalid "path" parameter (must be a non-empty string).' };
+	}
+
+	const rawContent = obj.content;
+	if (typeof rawContent !== "string") {
+		return {
+			error: 'Error: Missing or invalid "content" parameter (must be a string).',
+		};
+	}
+
+	let overwrite = false;
+	if (obj.overwrite !== undefined) {
+		if (typeof obj.overwrite !== "boolean") {
+			return { error: 'Error: Invalid "overwrite" parameter (must be a boolean).' };
+		}
+		overwrite = obj.overwrite;
+	}
+
+	return { path: rawPath, content: rawContent, overwrite };
+}
+
+/**
+ * Factory: create a write_file ToolContract bound to a working directory.
+ * The working directory is injected so the tool is testable.
+ */
+export function createWriteFileTool(workingDirectory: string): ToolContract {
+	const workdir = resolve(workingDirectory);
+
+	return {
+		name: "write_file",
+		description:
+			"Write a whole file to disk. " +
+			"By default, creates a new file; errors if it already exists. " +
+			"Set overwrite: true to replace an existing file (errors if the file does not exist). " +
+			"Parent directories are NOT auto-created — the parent must already exist.",
+		parameters: {
+			type: "object",
+			properties: {
+				path: {
+					type: "string",
+					description: "Path to the file, relative to the working directory.",
+				},
+				content: {
+					type: "string",
+					description: "The full content to write to the file.",
+				},
+				overwrite: {
+					type: "boolean",
+					description:
+						"When false/unset: creates a new file (errors if it already exists). " +
+						"When true: replaces an existing file (errors if it does not exist).",
+					default: false,
+				},
+			},
+			required: ["path", "content"],
+		},
+		concurrencySafe: false,
+		async execute(args: unknown, ctx): Promise<ToolResult> {
+			const validated = validateArgs(args);
+			if ("error" in validated) {
+				return { content: validated.error, isError: true };
+			}
+
+			const { path: relPath, content, overwrite } = validated;
+
+			const effectiveBase = ctx.cwd ? resolve(ctx.cwd) : workdir;
+			const resolvedPath = resolve(effectiveBase, relPath);
+
+			if (!isPathWithinWorkdir(resolvedPath, effectiveBase)) {
+				return {
+					content: `Error: Path "${relPath}" is outside the working directory.`,
+					isError: true,
+				};
+			}
+
+			// Symlink hardening: realpath the parent directory and the base, then re-check.
+			let realParent: string;
+			let realBase: string;
+			try {
+				const parentDir = dirname(resolvedPath);
+				[realParent, realBase] = await Promise.all([realpath(parentDir), realpath(effectiveBase)]);
+			} catch (err: unknown) {
+				const code = (err as NodeJS.ErrnoException).code;
+				if (code === "ENOENT") {
+					return {
+						content: `Error: Parent directory for "${relPath}" does not exist.`,
+						isError: true,
+					};
+				}
+				return {
+					content: `Error resolving path: ${err instanceof Error ? err.message : String(err)}`,
+					isError: true,
+				};
+			}
+
+			const realResolvedPath = realParent + sep + resolvedPath.split(sep).at(-1);
+			if (!isPathWithinWorkdir(realResolvedPath, realBase)) {
+				return {
+					content: `Error: Path "${relPath}" is outside the working directory.`,
+					isError: true,
+				};
+			}
+
+			// If the resolved path itself is a symlink, verify the target is contained.
+			try {
+				const linkStat = await lstat(resolvedPath);
+				if (linkStat.isSymbolicLink()) {
+					const linkTarget = await readlink(resolvedPath);
+					const resolvedTarget = resolve(dirname(resolvedPath), linkTarget);
+					if (!isPathWithinWorkdir(resolvedTarget, realBase)) {
+						return {
+							content: `Error: Path "${relPath}" is outside the working directory.`,
+							isError: true,
+						};
+					}
+				}
+			} catch (err: unknown) {
+				const code = (err as NodeJS.ErrnoException).code;
+				if (code !== "ENOENT") {
+					return {
+						content: `Error checking path: ${err instanceof Error ? err.message : String(err)}`,
+						isError: true,
+					};
+				}
+			}
+
+			// Check existence.
+			let fileExists = false;
+			try {
+				await access(resolvedPath);
+				fileExists = true;
+			} catch (err: unknown) {
+				const code = (err as NodeJS.ErrnoException).code;
+				if (code !== "ENOENT") {
+					return {
+						content: `Error checking file: ${err instanceof Error ? err.message : String(err)}`,
+						isError: true,
+					};
+				}
+			}
+
+			// Pure decision.
+			const decision = decideOverwrite(fileExists, overwrite);
+			if (typeof decision === "object") {
+				return { content: decision.error, isError: true };
+			}
+
+			// Verify it's not a directory.
+			if (fileExists) {
+				const pathStat = await stat(resolvedPath);
+				if (pathStat.isDirectory()) {
+					return {
+						content: `Error: "${relPath}" is a directory, not a file.`,
+						isError: true,
+					};
+				}
+			}
+
+			// Write the file.
+			try {
+				await writeFile(resolvedPath, content, "utf8");
+			} catch (err: unknown) {
+				return {
+					content: `Error writing file: ${err instanceof Error ? err.message : String(err)}`,
+					isError: true,
+				};
+			}
+
+			const action = decision === "create" ? "Created" : "Overwrote";
+			return { content: `${action} "${relPath}" (${content.length} bytes).` };
+		},
+	};
+}