Add GROK.md generator — micro-faf placeholder pattern

[Wolfe-Jam]

Gap

src/interop/ currently ships 4 vendor-md generators — agents.ts, claude.ts, cursorrules.ts, gemini.ts. The fifth — GROK.md — is missing.

Memory entry faf-cli-interop-vendor-md-generation-ready.md lists Grok as a supported vendor; the code doesn't match the doctrine.

Scope

One new file: src/interop/grok.ts, following the micro-faf placeholder pattern:

• Canonical 2-line FAF stamp at the top (per cross-ai-2-line-meta-stamp.md)
• Minimal body — name, one-line description, pointer to project.faf
• No attempt to replicate full CLAUDE.md content — just enough for Grok agents to find their bearings when grepping the repo

Acts as a discoverable surface; the structured truth stays in project.faf.

Why now

• Closes the 5-vendor matrix (Claude / Gemini / Cursor / Agents / Grok)
• Symmetric with the existing 4 generators — same fafMetaTag(data) 2-line stamp, same lines.push(...) pattern
• Resolves a documented memory↔code mismatch
• Adjacent to the existing FAF↔xAI / grok-faf-mcp thread

Affected

• New: src/interop/grok.ts
• Wire-up: wherever agents.ts / gemini.ts are registered (likely src/commands/export.ts)
• Test: tests/interop/grok.test.ts — stamp shape + content invariants

Out of scope

• Full Grok-specific prose surface (deferred — placeholder first, content later if Grok evolves a richer convention)