feat(ssh): wave 2 — route filesystem/shell tools behind ExecBackend

Wave 2 of transparent SSH support (4 parallel owner-agents on disjoint
tool packages). The tools now resolve an ExecBackend per-call from
ctx.computerId and call backend.spawn / backend.readFile / etc. instead of
node:fs and node:child_process directly — so they are transport-agnostic
(local now; remote over SSH later, transparent to the agent). Still LOCAL-ONLY
this wave (computerId always undefined -> LocalExecBackend, behavior-identical).

- tool-shell: factory takes resolveBackend; execute calls backend.spawn.
  spawn.ts DELETED (realSpawn was a verbatim duplicate of exec-backend's
  LocalExecBackend.spawn — logic moved to the sanctioned shared package).
  manifest dependsOn:[exec-backend]; host.getService at activation.
- tool-read-file: readFile/stat/readdir -> backend.* (pure logic untouched;
  ENOENT .code branches kept).
- tool-write-file: exists/stat/writeFile -> backend.* (pure logic untouched).
- tool-edit-file: readFile/writeFile -> backend.* + forward-compatible REMOTE
  diagnostics skip (ctx.computerId set -> skip LSP, return empty — plan §6.1;
  local path byte-identical to today). LSP lookup stays lazy.
- orchestrator: pre-wired @dispatch/exec-backend dep into the 4 tool
  package.jsons + bun install (build/config, my lane) so isolated verify
  resolved cleanly; agents added the ../exec-backend tsconfig ref.

Verified: tsc -b EXIT 0, biome clean, 1599 vitest pass (was 1592).
Refs: notes/ssh-support-plan.md (decisions §0.5/§13). No merge or push.

1 files changed, 4 insertions, 2 deletions

diff --git a/tasks.md b/tasks.md
index 4bbea5d..f785d4e 100644
--- a/tasks.md
+++ b/tasks.md
@@ -21,8 +21,10 @@ owner-agents on disjoint packages).
       `conversation-store` (contract fan-out: defaultComputerId + getEffectiveComputer
       + per-conv computerId get/set/clear). `tsc -b` EXIT 0, biome clean, **1592 vitest**
       (was 1549, +43).
-- [ ] **Wave 2** (parallel): refactor `tool-shell`/`read-file`/`write-file`/
-      `edit-file` behind `ExecBackend` (local-only still).
+- [x] **Wave 2** (parallel): refactor `tool-shell`/`read-file`/`write-file`/
+      `edit-file` behind `ExecBackend` (local-only; spawn.ts deleted — logic moved
+      to exec-backend; edit_file gains forward-compatible remote-diagnostics skip).
+      `tsc -b` EXIT 0, biome clean, **1599 vitest** (was 1592).
 - [ ] **Wave 3**: `conversation-store` (defaultComputerId + getEffectiveComputer)
       + `session-orchestrator` (resolve + thread computerId; drop lsp/mcp when
       remote) + `transport-contract` (computerId on ChatRequest + computer types).