Require phase-scoped Codex goals

[yvette-carlisle]

Summary

• remove the project-level codex.goal_support config surface
• require retained lane app-server goal methods instead of falling back to non-goal continuation
• align app-server/runtime/workflow docs and example config with mandatory phase-scoped goals
• drop old phase_goal_unavailable harness signal handling so the removed fallback no longer remains a first-class path

Recovery note

XY-878 attempt 2 produced the intended patch and passed validation, but Decodex classified the lane as retryable failure/no-effective-diff after the phase goal completed. This PR is the manual recovery path for that retained lane.

Validation

• cargo make fmt
• cargo test -p decodex goal --all-features
• rg -n "goal_support|ProjectCodexGoalSupportMode|phase_goal_required|phase_goal_unavailable|fallback.*goal|goal.*fallback" README.md docs apps/decodex/src decodex.example.toml plugins/decodex || true returned no matches
• semantic drift audit helper run; related goal-support/fallback terms reviewed as clean, broad signal/schema/fallback hits were unrelated false positives
• cargo make lint-fix
• cargo make test (1027 passed, 1 leaky, 1 skipped)
• cargo run -p decodex --bin decodex -- probe stdio://

Linear: XY-878