Add --plugin-dir and show what the cache saved

[icntswm]

What

• --plugin-dir <dir> (repeatable) tests a Claude Code plugin from its source directory: run, check, lint, list, init and gen load it as claude --plugin-dir does, with plugin:skill names and its commands. A source plugin replaces an installed one of the same name.
• check validates plugin:name for plugins loaded from --plugin-dir; installed plugin names are still skipped.
• Plugin manifests are read as Claude Code reads them: skills (path or array) adds to skills/, commands (path, array or object map) replaces commands/, a root SKILL.md alone is a single-skill plugin. This also fixes installed plugins.
• The cache fingerprint includes plugin docs from --plugin-dir and each doc's description (inline manifest commands have no frontmatter). Existing caches rerun once.
• Action: new plugin-dir input; .claude-plugin/plugin.json at the repository root is detected automatically. Plugin mode uses an empty isolated config dir. Env set by an earlier call of the action in the same job is cleared.
• Cached cases report what they saved: 2 cached (saved $0.21) in the terminal and Markdown, summary.savedUsd in JSON.

Checked

• npm run lint, npm test: 353 passed.
• Real run with --model haiku on a sample plugin: list shows tzkit:tz-convert and tzkit:standup, check catches tzkit:tz-convrt, both cases pass, the rerun is fully cached with saved $0.21.