chore: remove legacy in-repo docs tree

[nicholasdbrady]

Why now

taskbean/docs PR #1 is merged as commit 8b5dc828b009fa233f41e7eb305e509743a7219d, Mintlify is linked to taskbean/docs main (root docs.json), and the deployment reports Live at https://docs.taskbean.ai. Verified cutover evidence:

• Production /, /quickstart, sitemap, robots, llms.txt, llms-full.txt, and /.well-known/mcp all return 200.
• MCP JSON-RPC initialize POST returns 200 and identifies Taskbean v1.0.0.
• Legacy path /getting-started/installation redirects to /cli/installation with 200.
• Hosted desktop/mobile navigation and representative pages render without horizontal overflow.
• Production mint score is 99/A; remaining findings are non-blocking.
• PR-preview integration remains unproven but does not block removing the legacy in-repo copy — it isn't used by anything in this repo.

With taskbean/docs live and domain-verified, this repository's docs/ tree is no longer canonical and is safe to delete.

What changed

• Deleted the entire docs/ directory (Mintlify source, assets, docs.json).
• Kept docs.html unchanged — it still redirects to https://docs.taskbean.ai.
• Updated README.md, AGENTS.md, and .github/agents/taskbean-frontend-engineer.agent.md so none describe docs/ as a temporary migration source or local canonical docs; they now state canonical documentation is exclusively taskbean/docs and ordinary site work must not recreate a local docs/ tree.
• Removed the now-unused .mintlify/ entry from .gitignore (local Mintlify preview cache for the deleted tree).
• No marketing copy, design, assets outside docs/, or production site behavior changed.

Validation

• node scripts/validate-site.mjs → passed for 4 pages.
• node scripts/validate-agent-team.mjs → passed for 5 profiles and 6 routing cases.
• git diff --check → clean.
• Confirmed all surviving outbound docs links target https://docs.taskbean.ai and docs.html's meta-refresh/JS redirect still target docs.taskbean.ai.

Residual risk

• PR-preview integration for taskbean/docs is not yet proven end-to-end; unrelated to this removal since this repo never depended on it.

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