release: prepare v5.0.0

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Summary

• Remove the public OMX compatibility profile, selector arguments, generated OMX spec, and runtime marker-based profile switching; new installs converge on the single complete full profile.
• Absorb the runtime-independent OMX orchestration strengths into the full core as a bounded Codex-native subagent leadership contract while retaining one-way migration for the immediately previous dual-profile manifest.
• Fix update-time status-line ownership retention, malformed legacy profile validation, perf-baseline --slo sandbox cleanup, and the marketplace dual-surface E2E expectation.
• Synchronize the package, plugin, spec, and installer version to 5.0.0 and document migration plus rollback to immutable 4.25.4.

Verification

• npm run check: all Node and Bash suites passed, including 382/382 hook smoke checks; ShellCheck passed; live guard verified 24 real Codex surfaces unchanged.
• node scripts/version-cascade-check.js --json: no stale same-major prose token.
• node scripts/spec-source.js --check: generated full spec matches canonical fragments.
• npm pack isolated check: 112 entries; identity and required paths verified; no removed OMX spec, tests, state, tasks, or .env paths.
• Isolated CODEX_HOME install, SessionStart, update, doctor, and uninstall journey passed; former OMX marker, user guidance, custom config, and foreign hook remained preserved.