fix(agent): preserve pre-written claude-user-request.txt in agent mode

[syf2211]

Summary

Agent mode no longer wipes the entire claude-prompts directory. It only removes claude-prompt.txt, the file agent mode owns and rewrites.

Motivation

Fixes #1427. Composite workflows can pre-write claude-user-request.txt in an earlier step so the SDK takes the multi-block path and processes slash commands. PR #1288 added an unconditional rm -rf on the prompt directory, which deleted that pre-written file before the SDK ran.

Changes

• prepareAgentMode: replace directory rm -rf with targeted removal of claude-prompt.txt only
• Add regression test ensuring a pre-written claude-user-request.txt survives agent prepare

Tests

• bun test test/modes/agent.test.ts — 7/7 passed
• bun run typecheck — passed
• bun run format:check — passed
• composer-2.5 review: APPROVE

Notes

Tag mode still performs full directory cleanup in create-prompt (it writes both files). Agent mode intentionally preserves claude-user-request.txt for composite workflows. On non-ephemeral self-hosted runners without a pre-write step, a stale user-request file could theoretically persist if RUNNER_TEMP is not cleared between jobs — same class of concern noted in #1287, traded off here to unblock composite slash-command workflows.

[tarunag10]

This looks like the lowest-blast-radius fix for #1427 to me.

I had explored an alternate first-class user_request input in #1434, but this PR better preserves the existing composite-action behavior that regressed in v1.0.146: pre-writing claude-user-request.txt before invoking agent mode.

The main validation I would want is exactly what this tests: agent mode rewrites its owned claude-prompt.txt while leaving a pre-written claude-user-request.txt intact, so the SDK still takes the multi-block path for slash-command processing.