feat(executor): bring GeminiRunner to sandbox-flow parity with Claude

All coding tasks now follow the same flow regardless of runner: when
project_dir is set, the agent runs in a temp clone, not in the user's
working tree. On success, edits are autocommitted and pushed back to
origin/master and the sandbox is removed. On failure or BLOCKED, the
sandbox is preserved and its path surfaces in the error / BlockedError
so the user can inspect partial work or resume in place.

Before this commit, GeminiRunner.Run set cmd.Dir to project_dir
directly, so an agent run could leave half-done edits in the user's
working tree with no rollback. ClaudeRunner has had the full sandbox
flow for a while; this commit closes the gap.

Reused the existing package-level helpers from claude.go verbatim:
setupSandbox, teardownSandbox, sandboxCloneSource, gitSafe, plus the
resume/stale-sandbox/blocked-error patterns. No new shared abstraction
needed — same package.

LocalRunner intentionally not changed. The OpenAI chat path has no
tool use, so the agent can't edit files; sandbox would be theater.

Tests (6 new):
- Run_ProjectDir_RunsInSandbox: cwd captured by fake binary is a
  sandbox path, not project_dir.
- Run_BlockedError_IncludesSandboxDir: when question.json appears,
  BlockedError.SandboxDir is set and the dir exists.
- Run_ExecError_PreservesSandbox: failing exit wraps error with
  "(sandbox preserved at <path>)" and the path exists on disk.
- Run_ResumeUsesStoredSandboxDir: ResumeSessionID + SandboxDir →
  runs in that dir without re-cloning.
- Run_StaleSandboxDir_ClonesAfresh: resume pointing at missing
  dir falls back to a fresh clone from project_dir.
- Run_NoProjectDir_SkipsSandbox: tasks without project_dir don't
  trigger sandbox setup.

https://claude.ai/code/session_017Edeq947TpSm1vQTxMhi1J

0 files changed, 0 insertions, 0 deletions