feat(codex): add live skill shadow preflight

[slowdini]

Summary

• add the named codex-skills shadow preflight for repository, user, admin, and enabled plugin skills
• route build banners and aggregate validity warnings through harness-specific formatters while preserving the legacy plugin-shadow.json artifact
• allow Codex --bootstrap with --no-stage while retaining the --stage-name restriction
• update the descriptor schema, harness template, support table, and Codex implementation notes

Before / after

Before, Codex runs could not warn when a live skill contaminated the treatment or control arm, and --bootstrap --no-stage was rejected. After, Codex reports live collisions with Codex-specific remediation and supports bootstrap injection independently of native staging.

Detection limits

Bundled Codex system skills remain outside the preflight because Codex does not currently expose a stable enumeration mechanism for them. Missing directories, malformed skills, and unavailable plugin-list output fail open without suppressing other scan sources.

Verification

• cargo fmt --check
• cargo test — 466 unit, 99 CLI, and 79 run tests passed
• cargo clippy --all-targets -- -D warnings
• git diff --check

Follow-up to #113.