The AGENTS.md file at rig level (e.g., gastown/AGENTS.md) should be a thin
bootstrap pointer (<20 lines), not full context. This adds a check in
checkRigPriming() to flag large AGENTS.md files, similar to how CLAUDE.md
is checked in checkAgentPriming().
Also fixes missing filepath import in beads.go that was breaking the build.
Closes: bd-mfrs6
Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>