[#640] Sub-3: Add /llms.txt endpoint + direct Build tab link

[realproject7]

Parent: #640

Scope

Add machine-readable integration info for AI agents and support direct linking to the Build tab.

1. /llms.txt endpoint

Create `src/app/llms.txt/route.ts` — plain-text response with:

• CLI install command and available commands
• API endpoints with request/response formats
• Contract addresses (StoryFactory, ERC-8004, ZapPlotLinkV2)
• Chain info (Base, chainId 8453)
• RPC URL

2. "Copy llms.txt link" button on Build tab

Add a button in `AgentBuild.tsx` that copies `https://plotlink.xyz/llms.txt\` to clipboard.

3. Direct link to Build tab

Support `plotlink.xyz/agents?tab=build` via URL search params. Currently tabs are client-side state only — read `searchParams` on mount to set initial tab.

Files to modify

• New: `src/app/llms.txt/route.ts`
• `src/components/AgentBuild.tsx` — add llms.txt button
• `src/app/agents/page.tsx` — read `?tab=` search param

Branch

`task/640c-llms-txt`

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Self-Verification (T3)

• Run npm run dev, visit http://localhost:3000/llms.txt — verify plain text response with integration info
• Verify response includes: CLI commands, API endpoints, contract addresses, chain info
• Visit /agents?tab=build — verify Build tab is auto-selected
• Visit /agents?tab=dashboard — verify Dashboard tab is auto-selected
• Click the 'Copy llms.txt link' button — verify clipboard contains correct URL
• Run npm run build — no errors