feat: add agent user_request input

[tarunag10]

Summary

Adds a first-class user_request input for agent mode and writes it to claude-user-request.txt after prompt-dir cleanup.

Fixes #1427.

Root cause

Agent mode currently clears ${RUNNER_TEMP}/claude-prompts before writing claude-prompt.txt. That correctly removes stale prompt files, but it also deletes claude-user-request.txt files that composite actions intentionally pre-write before invoking this action.

The base SDK runner already supports claude-user-request.txt: when it exists alongside the prompt file, it sends a multi-block user message so Claude Code can process slash commands from the user request. After the cleanup change, downstream composite actions no longer had a supported point to provide that file.

Changes

• Adds user_request to the top-level action inputs.
• Exports it as USER_REQUEST and parses it into context.inputs.userRequest.
• In agent mode, writes claude-user-request.txt after stale prompt files are cleared.
• Keeps existing prompt cleanup behavior, so stale files from prior invocations are still removed.
• Adds a regression test that starts with a stale user request file, runs agent mode, and verifies the new request replaces it.

Validation

• ./node_modules/.bin/prettier --check .
• bun test test/modes/agent.test.ts test/github-context.test.ts base-action/test/run-claude-sdk.test.ts
• bun run typecheck