fix: clear stale claude-prompts dir before each write

[kyungilpark]

Fixes #1287.

Problem

${RUNNER_TEMP}/claude-prompts/ is shared across jobs on non-ephemeral self-hosted runners — the documented RUNNER_TEMP cleanup contract is not honored there. In particular, claude-user-request.txt is only written by create-prompt when a user request exists; in agent mode it is left untouched, so a stale value left by an earlier mention-mode job in repo A leaks into a later agent-mode job in repo B running on the same runner agent.

We observed this happen on ~40% of agent-mode review jobs landing on an affected runner agent over a 5-day window — every affected review surfaced an unrelated stale question from a much-earlier mention-mode job in another repo.

Issue #1287 has the full reproducer, evidence, and root-cause analysis.

Fix

Add await rm(dir, { recursive: true, force: true }) before each await mkdir(...) in both write sites:

• src/create-prompt/index.ts (tag/comment mode — writes claude-prompt.txt and claude-user-request.txt)
• src/modes/agent/index.ts (agent mode — writes claude-prompt.txt)

Each invocation now starts with a guaranteed-clean directory.

Side effect: extracted the shared path into a promptDir local in both files, eliminating four duplicated \${process.env.RUNNER_TEMP || "/tmp"}/claude-prompts literals.

Why this approach

Considered alternatives (per #1287):

1. Per-invocation subdirectory keyed by ${GITHUB_RUN_ID}-${GITHUB_RUN_ATTEMPT}-${GITHUB_JOB}.
   Cleaner isolation but requires changes at every consumer (src/entrypoints/run.ts:269 reads ${RUNNER_TEMP}/claude-prompts/claude-prompt.txt), bigger blast radius.
2. Cleanup at the start (this PR). One-line guard, idempotent, no consumer changes, handles every existing and future file in the directory uniformly.
3. Always-write claude-user-request.txt (sentinel). Fixes the specific leak but leaves the general cross-job staleness intact for any future file added to the directory.

Option 2 is the smallest, least invasive change and self-heals on every invocation.

Tests

The fs writes here are not covered by existing tests (test/create-prompt.test.ts, test/modes/agent.test.ts neither mock nor exercise the mkdir/writeFile path). I did not add new tests to keep the patch focused; happy to add an integration-style test that:

• creates a stale claude-user-request.txt in RUNNER_TEMP/claude-prompts/
• runs through createPrompt with a context that does not produce a user request
• asserts the stale file is gone after invocation

— if reviewers want it.

Verification

Manual reproducer from #1287 was used to confirm the leak exists on v1.0.67. After applying this patch in a downstream wrapping composite (mirroring the same rm-then-mkdir shape via shell), the leak no longer manifests.