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, 5 insertions, 0 deletions

diff --git a/packages/exec-backend/src/index.ts b/packages/exec-backend/src/index.ts
new file mode 100644
index 0000000..3135f79
--- /dev/null
+++ b/packages/exec-backend/src/index.ts
@@ -0,0 +1,5 @@
+export type { DirEntry, ExecBackend, ExecResult, SpawnParams, StatResult } from "./backend.js";
+export { createExecBackendExtension, manifest } from "./extension.js";
+export { createLocalExecBackend, localExecBackend } from "./local.js";
+export type { ExecBackendResolver } from "./service.js";
+export { execBackendHandle } from "./service.js";