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.

3 files changed, 72 insertions, 0 deletions

diff --git a/packages/kernel/src/runtime/dispatch.ts b/packages/kernel/src/runtime/dispatch.ts
index e0be1b4..01f0043 100644
--- a/packages/kernel/src/runtime/dispatch.ts
+++ b/packages/kernel/src/runtime/dispatch.ts
@@ -18,6 +18,7 @@ export async function executeToolCall(
 	turnId: string,
 	toolSpan?: Span,
 	cwd?: string,
+	computerId?: string,
 ): Promise<ToolResult> {
 	if (tool === undefined) {
 		return { content: `Unknown tool: ${call.name}`, isError: true };
@@ -34,6 +35,7 @@ export async function executeToolCall(
 		log: toolSpan?.log ?? createNoopLogger(),
 		conversationId,
 		...(cwd !== undefined ? { cwd } : {}),
+		...(computerId !== undefined ? { computerId } : {}),
 	};
 	// Race the tool's execute promise against the abort signal so a tool
 	// that hangs (ignores ctx.signal, or blocks on something the signal
@@ -74,6 +76,7 @@ export function createStepDispatcher(
 	turnId: string,
 	toolSpans: Map<string, Span>,
 	cwd?: string,
+	computerId?: string,
 ): StepDispatcher {
 	let activeCount = 0;
 	let unsafeRunning = false;
@@ -112,6 +115,7 @@ export function createStepDispatcher(
 			turnId,
 			tcSpan,
 			cwd,
+			computerId,
 		);
 		activeCount--;
 		if (entry.tool?.concurrencySafe === false) unsafeRunning = false;
diff --git a/packages/kernel/src/runtime/run-turn.test.ts b/packages/kernel/src/runtime/run-turn.test.ts
index 0d4c59d..08d3055 100644
--- a/packages/kernel/src/runtime/run-turn.test.ts
+++ b/packages/kernel/src/runtime/run-turn.test.ts
@@ -835,6 +835,71 @@ describe("runTurn", () => {
 		expect(capturedCwd).toBeUndefined();
 	});
 
+	it("forwards computerId from RunTurnInput to ToolExecuteContext", async () => {
+		let capturedComputerId: string | undefined = "SENTINEL_NOT_SET";
+
+		const tool = createFakeTool("computercheck", async (_input, ctx) => {
+			capturedComputerId = ctx.computerId;
+			return { content: "ok" };
+		});
+
+		const provider = createFakeProvider([
+			[
+				{ type: "tool-call", toolCallId: "tc1", toolName: "computercheck", input: {} },
+				{ type: "finish", reason: "tool-calls" },
+			],
+			[
+				{ type: "text-delta", delta: "done" },
+				{ type: "finish", reason: "stop" },
+			],
+		]);
+
+		await runTurn({
+			provider,
+			messages: [userMessage],
+			tools: [tool],
+			dispatch: { maxConcurrent: 1, eager: false },
+			conversationId: "tab-test",
+			turnId: "turn-test",
+			emit: () => {},
+			computerId: "ssh-host-alias",
+		});
+
+		expect(capturedComputerId).toBe("ssh-host-alias");
+	});
+
+	it("forwards undefined computerId when RunTurnInput has no computerId", async () => {
+		let capturedComputerId: string | undefined = "SENTINEL_NOT_SET";
+
+		const tool = createFakeTool("computercheck", async (_input, ctx) => {
+			capturedComputerId = ctx.computerId;
+			return { content: "ok" };
+		});
+
+		const provider = createFakeProvider([
+			[
+				{ type: "tool-call", toolCallId: "tc1", toolName: "computercheck", input: {} },
+				{ type: "finish", reason: "tool-calls" },
+			],
+			[
+				{ type: "text-delta", delta: "done" },
+				{ type: "finish", reason: "stop" },
+			],
+		]);
+
+		await runTurn({
+			provider,
+			messages: [userMessage],
+			tools: [tool],
+			dispatch: { maxConcurrent: 1, eager: false },
+			conversationId: "tab-test",
+			turnId: "turn-test",
+			emit: () => {},
+		});
+
+		expect(capturedComputerId).toBeUndefined();
+	});
+
 	it("aggregates usage across multiple steps", async () => {
 		const provider = createFakeProvider([
 			[
diff --git a/packages/kernel/src/runtime/run-turn.ts b/packages/kernel/src/runtime/run-turn.ts
index f5d80d3..940c77f 100644
--- a/packages/kernel/src/runtime/run-turn.ts
+++ b/packages/kernel/src/runtime/run-turn.ts
@@ -117,6 +117,7 @@ interface StepContext {
 	readonly turnSpan: Span | undefined;
 	readonly toolSpans: Map<string, Span>;
 	readonly cwd: string | undefined;
+	readonly computerId: string | undefined;
 	readonly now: (() => number) | undefined;
 	/** Per-turn provider options (model, systemPrompt, …) threaded to stream(). */
 	readonly providerOpts: ProviderStreamOptions | undefined;
@@ -295,6 +296,7 @@ async function executeStep(ctx: StepContext): Promise<StepResult> {
 		ctx.turnId,
 		ctx.toolSpans,
 		ctx.cwd,
+		ctx.computerId,
 	);
 
 	const timing: TimingState = {
@@ -522,6 +524,7 @@ export async function runTurn(input: RunTurnInput): Promise<RunTurnResult> {
 				turnSpan,
 				toolSpans,
 				cwd: input.cwd,
+				computerId: input.computerId,
 				now,
 				providerOpts: input.providerOpts,
 			});