docs(search): relocate /search setup to docs and fix dead URLs

[quiet-node]

Summary

• Moves the /search Docker setup out of the README "Getting Started" flow into a new ## Setup section at the top of docs/agentic-search.md (anchor: #setup). Adds an inline roadmap note that out-of-box /search (native sidecar or bundled images) is planned, but today still requires cloning the repo.
• Replaces the now-removed README anchor #step-2-setup-the-search-sandbox-... everywhere it was linked: SandboxSetupCard.tsx (frontend setup-guide button), SandboxSetupCard.test.tsx (test assertion), src-tauri/src/search/types.rs (SandboxUnavailable user-facing error message). All now point to docs/agentic-search.md#setup.
• Updates src/config/commands.ts (generator source) so the /search limit prose links to the new setup anchor, and regenerates docs/commands.md via bun run generate:commands. The commandArtifacts.test.ts parity gate is preserved.

Test plan

• bun run validate-build passes with zero warnings / errors
• bun run test:all:coverage passes; line coverage remains at 100%
• bun run generate:commands is idempotent (regenerated output is byte-identical to the committed docs/commands.md)
• No other generated artifacts exist in the repo (audited via grep for Generated from / @generated / DO NOT EDIT); only docs/commands.md carries the marker
• Manual: trigger the /search pre-flight in the app with the sandbox stopped and click the "Setup Guide" link in the offline card — confirm it opens docs/agentic-search.md#setup in the browser