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, 4 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;