Add CLAUDE.md with repo guidance for AI assistants

[quicksilverj2]

Summary

Adds a CLAUDE.md at the repo root documenting how AI assistants should work in this Mintlify documentation site.

Key points covered:

• Scope boundary: this repo holds docs only; the PrivateInvest application source lives elsewhere — so requests to "fix bugs" or "add features" should be confirmed as doc edits, not code edits.
• Repository layout: docs.json + index.mdx + agent/, backend/, frontend/ subdirectories.
• Authoring conventions: required MDX frontmatter (title + description), Mintlify components (<Card>, <Columns>), absolute extension-less internal links (e.g. /backend/setup) — matching the fix in a378ae5.
• docs.json registration: new pages must be added to navigation.tabs[].groups[].pages or they won't appear in the sidebar.
• Product terminology: consistent role names (ADMIN/COMPLIANCE/IFA/DEAL_ORIGINATOR), ports (4000/4001/4002), and stack details.
• Git workflow for this environment: designated branch, draft PR after push, MCP-only GitHub access scoped to quicksilverj2/docs.
• Anti-patterns: don't invent product facts, don't reintroduce the planning/status docs removed in 6c45942.

Test plan

• Confirm the file renders correctly on GitHub.
• Sanity-check that nothing in docs.json or any .mdx was changed (this PR adds a single new file).

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