FE-1110: Consolidate executor sandbox port internals

[kostandinang]

Follow-up cleanup on top of #275. No behavior change.

What changed

• Drop dead cwd field from TestRunArgs — it was never read.
• Share one subprocess runner — extracted src/app/command-runner.ts so the git worktree port and test runner port stop duplicating spawn logic. Git spawn-error behavior preserved (covered by a new test).
• Share test fakes — port test doubles that were copied across three test files now live in src/executor/__tests__/fake-ports.ts.

Verification

npm run verify passes: 165 test files, 1111 tests, build ok.

[cursor]

PR Summary

Low Risk
Refactor and interface narrowing only; behavior preserved and covered by existing port and executor tests plus a new git spawn-error case.

Overview
Follow-up cleanup that deduplicates executor sandbox plumbing without changing cook run behavior.

Shared subprocess runner: New src/app/command-runner.ts centralizes spawn-based execution with a spawnError field on spawn failures. createGitWorktreePort and createTestRunnerPort both use it instead of copy-pasted runCommand helpers. Git failure messages now fall back to spawnError when stderr/stdout are empty (new test covers spawn git ENOENT).

Narrower test port contract: TestRunArgs drops unused cwd; verify still runs with cwd: worktreeDir. ingestTestResult and port tests call testRunner.run({ worktreeDir }) only.

Shared test doubles: createFakeGitWorktreePort / createFakeTestRunnerPort live in src/executor/__tests__/fake-ports.ts (git fake accepts an optional custom create); registry and worktree tests import them instead of local copies.

Also removes obsolete memory/REFACTOR.md planning doc.

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