chore: wire Graft as the agent context layer
Appends a marker-fenced Graft policy to the existing commandF engineering rules. Deterministic validators/tests remain authoritative; Graft is advisory repository context only. Requires zero-cost use and preserves Jev, Alibaba Open Code Review, CI, and security qualification. Full MCP/hooks wiring still requires local graft init; no local execution is claimed.
Summary by cubic
Adds a marker-fenced Graft policy to AGENTS.md, making Graft the default agent context layer for codebase navigation.
Graft output is advisory; deterministic validators, tests, and project policy remain authoritative. The policy requires local graft init and graft build usage, treats graft/ as a non-committed cache, and keeps usage zero-cost by forbidding paid model/API usage. Existing CI, Jev review, Alibaba Open Code Review, and security checks are unaffected.