Refresh public landing page and add rendered agent-instructions view

[HamptonMakes]

Why

The engine's default landing page is what any company deploying CoPlan shows employees at /, and what the open-source project shows the world — but it was a thin three-step page that under-sold the product. Worse, its "agent instructions" link pointed at GET /agent-instructions, which serves raw text/markdown and opens as an ugly wall of text in a browser, even though that endpoint is the front door we tell every human to hand to their agent.

What

Landing page (coplan/welcome/_default_landing)

• Rewrote the default landing partial to actually pitch the product, using language from README/CONCEPT (no invented testimonials, logos, or metrics): the humans comment, agents edit model, Google-Docs-style inline anchored commenting, immutable versions with full provenance, realtime collaboration, the plan lifecycle, and the semantic agent API.
• Added a CSS-only illustrative mock of the review loop (anchored highlight → reviewer comment → agent edit → new version) built from the same design tokens as the real plan UI, so it themes correctly in light and dark.
• CTAs adapt to auth state: signed-in users get "Browse plans"; anonymous visitors get "Sign in to get started" (when the host configures sign_in_path) — they're one auto-login away. Everyone gets "Connect your agent".
• Still rendered via config.landing_page_partial and the agents section still swaps independently via config.landing_agents_partial, so host overrides (e.g. coplan-square's sq agents skills add coplan) keep working. The default agents section now also shows the full instructions URL with a copy button.

Agent instructions (/agent-instructions)

• Browsers now get a styled HTML page: "Connect your AI agent" header, a prominent copy-to-clipboard chip with the raw instructions URL, a curl example, a link to the raw markdown, and the full instructions document rendered via the existing MarkdownHelper#render_markdown(interactive: false).
• The raw endpoint behavior is unchanged and load-bearing: every API response's X-Agent-Instructions header points here, so negotiation is deliberately conservative. HTML is served only when the Accept header leads with text/html (exactly what browsers and Turbo Drive send). curl's default Accept: */*, an absent Accept, and Accept: text/markdown all get text/markdown byte-identical to before (spec-asserted, and verified live with real curl). .md always forces raw; .html always forces the page. A string check on the raw header is intentional — Rails maps */* to HTML, so respond_to/request.format would misroute every agent.
• Signed-in visitors keep their normal nav chrome on the HTML page (optional user resolve, same approach as WelcomeController).

Plumbing

• New coplan--clipboard Stimulus controller (vanilla, auto-pinned via the existing pin_all_from), with "Copied!" feedback, a non-secure-context fallback, and label restore on disconnect so Turbo's cache never snapshots a stale state.
• Shared coplan/shared/_copy_url partial used by both the landing agents section and the instructions page.
• All new CSS uses the existing design tokens/BEM style in coplan/application.css; verified in both light and dark themes.

Testing

• New/updated request specs: anonymous landing render + CTAs, agents-section copy-URL element, content negotiation (markdown byte-identical across */* / text/markdown / no header; HTML for browser Accept; .md and .html overrides; signed-in nav chrome).
• bundle exec rspec: 954 examples, 0 failures.
• Verified live against a dev server: real curl (with and without Accept headers) gets identical raw markdown; browser screenshots of / and /agent-instructions in light and dark; copy button click shows "Copied!" and writes the URL to the clipboard.

🤖 Generated with Claude Code