feat(site): Claude Code plugin page

[benbaarber]

Adds a marketing page for the Claude Code plugin at /plugin/, with a Plugin nav entry between CLI and Visualizer.

Content

• Hero: the four commands, the seven supported harnesses, and the two-line marketplace install.
• Your sessions are a dataset — three /path:query showcases (last week's work, costliest sessions, workflow inefficiency) rendered as transcript-style Q&A blocks (new .qa CSS component, theme-aware).
• Share, link, resume — one section per command, each with its own example block; the three examples form a small narrative around one shared session.
• How it works — self-installing CLI, local normalization, explicit-upload-only.

The example answers are illustrative, but each question class was validated end-to-end: fresh agents given only the plugin's query command doc answered all of them against a real cache (1–6 query calls, 46s–2.6min each).

Notes

• Stacked on Plugin /path:resume + /path:link-pr; fixes that make projected Claude sessions resumable #150 (/path:resume + /path:link-pr are referenced by the page).
• CLAUDE.md's site-build note moves from 11 to 12 pages; no page-count assertion exists in the gates.
• Site builds clean; full quality gates pass.

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