feat(doctor): add cleanup check and --dry-run support

[SepineTam]

Motivation

• Provide a built-in cleanup for old timestamp-prefixed files under the project stata-mcp-folder to avoid accumulation of legacy logs/tmp/do files.
• Make cleanup configurable via project/env (CLEAN_LOG_DAYS) and safe to preview with a --dry-run option.

Description

• Add src/stata_mcp/utils/clean_log.py implementing scan_old_files() and clean_log_files() with support for 14-digit (%Y%m%d%H%M%S) and 20-digit (%Y%m%d%H%M%S%f) leading timestamps, non-recursive scanning, dry-run reporting, and per-file error accounting.
• Add Config.CLEAN_LOG_DAYS in src/stata_mcp/config.py that reads STATA_MCP__CLEAN_LOG_DAYS / config file / default -1 using existing _get_config_value semantics.
• Extend CLI in src/stata_mcp/cli/_parsers.py to accept --dry-run for the doctor subcommand and forward the flag through src/stata_mcp/cli/_handlers.py into run_doctor.
• Implement check_cleanup() in src/stata_mcp/utils/doctor.py, add _format_size() helper, register cleanup in AVAILABLE_CHECKS, and propagate dry_run through _all_checks() and run_doctor() to control scan vs delete behavior and reporting.

Testing

• Ran the test suite with PYTHONPATH=src pytest -q and all tests passed locally (10 passed, several warnings unrelated to changes).
• Verified code compiles with PYTHONPATH=src python -m compileall -q src with no errors.
• Exercised the new cleanup paths via unit test iterations during development (scan + dry-run + deletion flows) and the doctor flow was confirmed to forward --dry-run correctly.

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Codex Task

[cloudflare-workers-and-pages]

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