feat: add Codex support for PUA subcommands

[iF2007]

Summary

Add first-class Codex support for namespaced PUA commands such as $pua:p7, $pua:on, and $pua:survey, while keeping the legacy pua entrypoint working.

Design

This change treats codex/ as a Codex-facing export layer, not a second source of truth. Canonical skill behavior stays in skills/, while Codex installs only from exported artifacts under codex/.

A registry-driven install flow now defines exported names, install paths, canonical target mappings, and the fallback prompt. This makes the Codex integration deterministic, scriptable, and easy to extend as new subcommands are added.

Additional Fixes

Besides subcommand support, this also cleans up several consistency issues in the Codex packaging layer:

• removes metadata drift by deriving Codex exports from canonical skills instead of maintaining wrapper metadata separately
• replaces fragile manual symlink steps with a proper generate / verify / install / uninstall flow
• adds manifest-backed uninstall safety so cleanup only touches installer-managed artifacts
• preserves backward compatibility through the legacy pua entrypoint and /prompts:pua fallback routing
• makes future Codex subcommands additive instead of requiring hand-maintained install command lists

Verification

Verified with registry/export consistency checks and install round-trip tests, including isolated install/uninstall validation.

Closes #107