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, 65 insertions, 0 deletions

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([
 			[