fix(plugin): ship skills to Codex installs via a generated real-file mirror

[max-sixty]

Codex installs of the worktrunk plugin carried no skills. codex plugin add copies the plugin into $CODEX_HOME/plugins/cache/ via copy_dir_recursive (codex-rs core-plugins), which handles only regular files and directories — so the skills -> ../../skills symlink (and the nested reference/README.md link inside the tree) were silently dropped, and sessions load from that cache copy. Verified against codex-cli 0.144.1 both by reading the tagged sources and by installing this repo's plugin into a scratch CODEX_HOME: the installed root had no skills/ at all and codex debug prompt-input showed an empty plugin skill inventory.

This PR cuts the plugin's skills symlink over to a generated real-file mirror of the authored repo-root skills/, kept current by a new sync_plugin_skills_mirror stage in test_docs_are_in_sync (dereferences symlinks, deletes stale files, self-heals and fails on drift — same pattern as the other generated mirrors). Repo-root skills/ stays the authored home: Gemini hard-probes it at the extension root, the docs sync writes into it, and Windows checkouts read it. Reversing the symlink direction instead would put a symlink at the repo root, breaking Gemini's install copy and every Windows checkout — which also surfaces a latent bug this fixes: symlinks materialize as plain text files on Windows clones, so installs from a Windows checkout shipped no skills to Claude or Codex either.

It also drops the Codex manifest's skills: "./skills/" key: with no key, Codex scans <plugin-root>/skills/ by convention (default_skill_roots in codex-rs/core-plugins/src/loader.rs), and the explicit path resolves to the same directory, so the key was redundant — symmetric with the Claude manifest cutover in #3431.

For the reviewer:

• The 20 files under plugins/worktrunk/skills/ are the generated mirror (byte-identical to repo-root skills/; git stores shared blobs once). plugins/worktrunk/CLAUDE.md → "Plugin skills are a generated mirror" documents the rationale with codex-rs citations.
• sync_plugin_skills_mirror in tests/integration_tests/readme_sync.rs is the sync stage (Step 3b of the pipeline); it already proved itself once in this branch — merging main regenerated reference/{config,list}.md in the mirror.
• test_plugin_layout_is_consolidated now pins the mirror shape cross-platform (real directory, no symlinks anywhere under it), replacing the unix-only symlink assertion.

Verification: fresh scratch-CODEX_HOME install now carries both skills into the cache and codex debug prompt-input lists worktrunk and wt-switch-create; claude plugin validate passes and claude --plugin-dir … plugin details discovers Skills (2) plus all 8 hooks; Gemini's repo-root path is untouched. A 2×2 probe matrix (manifest key present/absent × real dir/symlink) confirmed key-absence changes nothing and symlinks ship nothing.

This was written by Claude Code on behalf of max

[worktrunk-bot]

Looks good — the mirror correctly dereferences the nested reference/README.md symlink into byte-identical real files, the sync runs after sync_skill_files so it mirrors fresh content, and test_plugin_layout_is_consolidated catches a symlink before recursing into it (is_symlink() check precedes the is_dir() push). Verified the manifest-key drop and the codex-cli citations against the described behavior.

One non-blocking note on the "self-heals on drift" property: sync_plugin_skills_mirror writes before it removes, so a same-path type flip — a reference/foo.md file becoming a foo.md/ directory (or vice versa) — would panic on create_dir_all/write rather than converge, and wouldn't self-heal on rerun. Effectively unreachable in this all-.md tree and it fails loudly with the offending path, so not worth restructuring the loop over — just flagging since the design aims for self-healing.