fix: enforce build verification gate + retry empty OpenRouter responses

[DeSennd]

Problem

ForgeGod has no enforced build/compile gate for compiled languages (C++/CMake, Rust, Go, etc.):

1. Closure gate checks execution, not success — agent.py tracks verification commands by substring-matching the command string, but never checks the exit code. A failing cmake --build (exit 2) satisfies the gate the same as a passing one.

2. No build gate in story finalization — loop.py _finalize_story_result runs lint (Python/TS only) → reviewer (LLM text, no subprocess) → taste → status = DONE. Nothing ever invokes a compiler.

3. C++ tools invisible to closure gate — VERIFICATION_COMMAND_MARKERS had no cmake, make, ctest, g++, clang++. Build commands were unrecognized.

4. OpenRouter null content crash — msg.get("content", "") returns None when JSON has "content": null (key exists with null value). This propagates to agent.py:373 where response_text.strip() crashes with AttributeError: 'NoneType' object has no attribute 'strip'. No retry — the router treats null as success.

Fix

1. config.py: Add VerifyConfig ([verify] section) with build_command, test_command, timeout_s, max_fail_lines.

2. loop.py: Add _run_verify_command helper + build/test gate in _finalize_story_result. Runs build_command as subprocess before marking DONE; non-zero exit routes story back to TODO with compiler output in error_log.

3. agent.py: Track bash exit codes via [exit code: N] regex. Block completion when last verification command failed. Reset on new writes.

4. agent.py: Add C++ build tools to VERIFICATION_COMMAND_MARKERS.

5. router.py: Rewrite _call_openrouter with retry loop (3 retries, 4s delay) for empty/null content. Normalize msg.get("content") or "" to fix NoneType crash.

Evidence

Observed on a 65-story C++/CMake project: 16 stories marked done with zero successful builds after P0-01. Every cmake --build returned exit 2 in commands.log but stories still closed as done. The NoneType crash burned 3+ story retry slots across P0-01, P0-04, P2-02.

Backward compatibility

• VerifyConfig defaults to build_command = "" (empty) — gate is skipped when no command configured, so Python/JS projects are unaffected.
• FileNotFoundError (toolchain missing) skips the gate rather than blocking.
• Router retry only adds delay on empty responses; successful responses return immediately.