feat(ssh): wave 1 — ExecBackend + computer data model + runtime threading

Wave 1 of transparent SSH support (parallel owner-agents on disjoint packages,
plus the orchestrator-authored kernel contract seam from wave 0):

- packages/wire: + Computer/ComputerEntry (read-only view over ~/.ssh/config
  Host aliases) + Workspace.defaultComputerId (string|null, null=local). Types
  only; 3 conformance tests.
- packages/exec-backend (NEW core extension): the ExecBackend abstraction
  (spawn + minimal fs surface) the bundled tools will program against instead
  of node:fs/child_process. LocalExecBackend wraps today's node calls
  (behavior-identical; node:fs-style .code errors). execBackendHandle +
  ExecBackendResolver (sync; computerId undefined -> local; set -> throws until
  the ssh package wires remote resolution in wave 5). 20 tests.
- packages/kernel (runtime only): thread computerId through dispatch.ts +
  run-turn.ts exactly as cwd is threaded (opaque, forwarded to
  ToolExecuteContext; absent = local = byte-identical to today). +2 tests.
- packages/conversation-store: computer (SSH alias) assignment + resolution
  mirroring cwd — WorkspaceRow.defaultComputerId + setWorkspaceDefaultComputerId
  + getComputerId/setComputerId/clearComputerId + getEffectiveComputer
  (override -> per-conv -> workspace default -> null/local). Fixes the 3
  Workspace literal sites the new required wire field broke. +18 tests.
- orchestrator: root tsconfig.json ref for exec-backend + bun install.

Verified: tsc -b EXIT 0, biome clean, 1592 vitest pass (was 1549, +43).
Refs: notes/ssh-support-plan.md (decisions §0.5/§13). No merge or push.

1 files changed, 63 insertions, 0 deletions

diff --git a/packages/exec-backend/src/backend.test.ts b/packages/exec-backend/src/backend.test.ts
new file mode 100644
index 0000000..30458e7
--- /dev/null
+++ b/packages/exec-backend/src/backend.test.ts
@@ -0,0 +1,63 @@
+import { describe, expect, it } from "vitest";
+import type { DirEntry, ExecBackend, ExecResult, SpawnParams, StatResult } from "./backend.js";
+
+/**
+ * ExecBackend type conformance — a fake backend satisfies the interface.
+ * (Pure compile-time + runtime check; zero internal mocks.)
+ */
+describe("ExecBackend type conformance", () => {
+	it("a minimal fake satisfies the ExecBackend interface", () => {
+		const fake: ExecBackend = {
+			spawn: async (_params: SpawnParams): Promise<ExecResult> => ({
+				exitCode: 0,
+				timedOut: false,
+				aborted: false,
+			}),
+			readFile: async (_path: string): Promise<string> => "",
+			writeFile: async (_path: string, _content: string): Promise<void> => {},
+			stat: async (_path: string): Promise<StatResult> => ({ isFile: true, isDirectory: false }),
+			readdir: async (_path: string): Promise<readonly DirEntry[]> => [],
+			exists: async (_path: string): Promise<boolean> => true,
+		};
+
+		// Runtime sanity: every method is present and callable.
+		expect(typeof fake.spawn).toBe("function");
+		expect(typeof fake.readFile).toBe("function");
+		expect(typeof fake.writeFile).toBe("function");
+		expect(typeof fake.stat).toBe("function");
+		expect(typeof fake.readdir).toBe("function");
+		expect(typeof fake.exists).toBe("function");
+	});
+
+	it("ExecResult is { exitCode, timedOut, aborted }", () => {
+		const result: ExecResult = { exitCode: null, timedOut: true, aborted: false };
+		expect(result.exitCode).toBeNull();
+		expect(result.timedOut).toBe(true);
+		expect(result.aborted).toBe(false);
+	});
+
+	it("SpawnParams carries the shell-tool seam fields", () => {
+		const params: SpawnParams = {
+			command: "echo",
+			cwd: "/tmp",
+			signal: new AbortController().signal,
+			timeout: 1000,
+			onOutput: () => {},
+		};
+		expect(params.command).toBe("echo");
+		expect(params.timeout).toBe(1000);
+	});
+
+	it("StatResult distinguishes file vs directory", () => {
+		const fileStat: StatResult = { isFile: true, isDirectory: false };
+		const dirStat: StatResult = { isFile: false, isDirectory: true };
+		expect(fileStat.isFile && !fileStat.isDirectory).toBe(true);
+		expect(!dirStat.isFile && dirStat.isDirectory).toBe(true);
+	});
+
+	it("DirEntry carries name + isDirectory", () => {
+		const entry: DirEntry = { name: "sub", isDirectory: true };
+		expect(entry.name).toBe("sub");
+		expect(entry.isDirectory).toBe(true);
+	});
+});