Enhance security & Code Quality

[daocha]

security: harden session_id/branch validation, workspace locking, sec)ret scrubbing, and add 29 tests

Security fixes:

• validate session_id from LLM JSONL output with strict regex; reject flag-like values (e.g. '--exec') before they reach subprocess args
• workspace-based asyncio.Lock (per project_folder, not per chat_id) so multiple chat IDs sharing a workspace are serialized; second concurrent call receives 'already running' message immediately
• validate branch names in checkout_branch() and prepare_branch() with a regex allowlist; reject names starting with '-'
• _scrub_secrets() redacts Telegram tokens, GitHub PATs, AWS keys, etc. from agent prose output before sending to Telegram; _sanitize_agent_error() strips absolute filesystem paths from error messages shown to users
• resolve symlinks in _path_within_project() so an in-project symlink pointing outside the workspace root is rejected by /commit path validation

Code quality:

• implement RotatingFileHandler (10 MB / 3 backups) in logging_utils.py — log_file config was previously ignored
• wrap all 5 portalocker.Lock sites in session_store.py with SessionStoreError; bot.py handles it with a user-friendly retry message
• watchdog thread kills the agent subprocess after AGENT_HARD_TIMEOUT_SECONDS (default 0 = disabled) — configurable for operators who need a hard cap; normal Copilot long-running jobs are unaffected
• _sanitize_agent_error() strips internal paths from error messages
• log OSError in diff_utils._read_snapshot_text() instead of silently swallowing it
• structured audit log entry at INFO level for every agent invocation (chat_id, project_folder, provider, truncated prompt)
• router/base.py: refactor _run_with_typing → _execute_with_typing to allow the workspace lock guard to wrap it cleanly
• .gitignore: explicitly list .env (was only covered by glob patterns before)

Tests (29 new across 5 files + 1 new file):

• test_agent_runner.py: _validate_session_id (valid, flag-like, special chars, empty), JSONL rejects flag session_id, JSONL accepts valid session_id
• test_git_utils.py: _validate_branch_name (valid, flag-like, special chars, too long), checkout_branch rejects invalid name, prepare_branch rejects invalid new_branch and origin_branch
• test_command_router.py: _path_within_project allows normal files, blocks symlink-to-dir outside, blocks symlink-to-file outside; workspace lock blocks concurrent call on same project, allows different projects concurrently
• test_session_store.py: load() and save() raise SessionStoreError on lock timeout (portalocker.LockException monkeypatched)
• test_session_runtime_security.py (new): _scrub_secrets redacts Telegram token, GitHub PAT, AWS key, leaves normal text unchanged, handles multiple secrets; _sanitize_agent_error redacts Unix path, Windows path, leaves relative paths and plain messages unchanged