Clarify approved-plan handoff and installer sync

[xi-kari]

Summary

• add an Approved High-Risk Plan Exception for crossframe-coder and sync handoff rules
• split dual-core routing conflict tests into missing vs present approved Deep Risk plan cases
• sync Trial Materials for new dual-core and golden implementation evals
• harden installer with install-plan preflight, custom recursive copy, stale-file pruning, and post-copy exact match checks
• add executable installer smoke test and GitHub Actions validation workflow
• update README and INSTALL documentation for local/CI validation and install safety semantics
• extend coder source-driven API implementation rules with a decision table and bad examples

Verification

• node scripts/test-install-adapters.mjs -> passed
• node scripts/validate-skill.mjs -> 719 PASS / 0 FAIL / 2 INFO
• node scripts/install-adapters.mjs --platform codex --force -> SKIP identical for both local skills on second run