XY-699: Add executable Codex app-server compatibility guard

[yvette-carlisle]

Summary

• Adds an executable app-server compatibility preflight check based on initialized userAgent and the locally verified Codex CLI range.
• Exposes compatibility status, observed Codex version, and supported versions in decodex probe stdio:// output.
• Pins docs-to-code compatibility with tests covering codex-cli 0.136.0, codex-cli 0.136.0-alpha.2, and unverified identities outside the support claim.

Verification

• cargo make fmt
• cargo make lint-fix (stable rerun applied 0 vstyle fixes)
• cargo make test (919 passed, 1 skipped)
• cargo run -p decodex --bin decodex -- probe stdio:// -> compatibility=supported, codex_version=codex-cli 0.136.0, output=PROBE_OK
• python3 playbook semantic_drift_audit.py equivalent helper reported no stale phrase hits
• git diff --check