docs: add AGENTS.md with Cursor Cloud dev environment instructions

[kellenff]

Summary

Sets up and verifies the development environment for yactt (a self-contained Go 1.26.5 code-intelligence MCP server + CLI) and captures durable, non-obvious startup/run guidance for future cloud agents in a new AGENTS.md.

No product code was changed — only AGENTS.md was added.

Environment

• Go 1.26.5 is already present and matches go.mod; the only runtime dependency (smacker/go-tree-sitter) is compiled in. No DB/web/queue services are required.
• Update script (dependency refresh only): go mod download.

Verification

All standard dev commands pass:

• ✅ go vet ./...
• ✅ go build -trimpath -o /tmp/yactt ./cmd/yactt
• ✅ go test ./... (all packages ok)
• ✅ CLI: yactt overview <path> (tree-sitter fallback, no LSP installed — expected)
• ✅ MCP HTTP end-to-end: initialize → tools/list (21 tools) → index_repository (4 files) → find_symbol / search_code
• ✅ gitleaks detect --source . (no leaks)
• ⚠️ govulncheck ./... (CI-pinned @v1.1.4 is too old to analyze Go 1.26 code — a tool-version limitation, not a code issue)

End-to-end demo log:

yactt_mcp_e2e_demo.log

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