fix: orchestration reliability, security, and retry quality (9 targeted fixes)

[dcondrey]

Summary

Nine targeted fixes identified by automated audit of the hot-path production code. All pass the full test suite (2391 passed, 4 skipped).

Resource safety

• parallel.py: check worktree_remove() result; log ERROR on failure instead of silently proceeding
• web_verify.py: clamp browser nav timeout to ≥1000ms — prevents negative timeout from long server startup causing immediate false-positive failure

LLM retry quality

• execute_mixin.py / gates_mixin.py: prefix error_logs with gate type ([BUILD], [TYPECHECK], [TEST]) so the model knows which gate to target on retry
• execute_mixin.py: cap reflections to last 3; trim error_logs to 20K chars — prevents context window crowding on hard tasks with many retries

Security

• mcp_gather.py: wrap MCP tool results in <tool_result name="..."> delimiters before injecting into the LLM prompt — blocks prompt-injection from malicious/compromised MCP servers
• providers.py: stop storing the OpenRouter API key as an instance attribute (self.api_key) — key no longer appears in tracebacks, repr(), or debugger captures

Gate correctness

• gatekeeper/__init__.py: log WARNING when mutation gate errors and silently SKIPs — distinguishes tool-unavailable-by-design (INFO) from unexpected gate failure (WARNING)

Infrastructure

• mcp_registry.py: reload cache from disk when file mtime changes (multi-process safety); clean up .tmp file in finally block on atomic flush failure

Carried forward from prior tier-run work (commits 6656fa5–7700710): topological sort, tasklist dep table, LLMCache eviction amortization, HTTP read bounds — these were in roadmap/tier-run but not captured in PR #11's merge.

Test plan

• CI passes (all py3.10–3.13 × ubuntu + macos)
• pytest tests/ -x -q — 2391 passed locally