[codex] Improve goal mode guidance

[bobleer]

Summary

• Tighten /goal synthesis so user-provided goal focus takes priority and generated goals stay execution-focused.
• Improve goal verification guidance by requiring concrete gaps, actionable recovery guidance, and verdict self-repair when the verifier returns invalid JSON.
• Include the just-finished final assistant response in verification context if the runtime context has not caught up yet.

Root Cause

The existing goal continuation display and verifier prompt could fall back to restating the goal instead of producing concrete recovery guidance. Verification also did not enforce contradictory verdicts such as achieved=true with remaining gaps.

Validation

• rustfmt --edition 2021 --check src/crates/core/src/agentic/goal_mode/mod.rs
• cargo test -p bitfun-core goal_mode -- --nocapture