Hardening/activation artifacts land in the canonical source: AGENTS.md scaffold is imported as a page and .gbrain-owner.json stays untracked

Summary

Two small artifacts of sources harden + managed mode end up inside the brain's own canonical source:

1. The durability scaffold is imported as a page. sources harden writes an AGENTS.md rules block into the source root; the next sync imports it as a page (agents), so the brain's own operating scaffold shows up in search results and in facts-absorb (which then skips it with writer_coordinator_required).
2. The ownership marker is untracked and unignored. Activation writes .gbrain-owner.json into the claimed worktree root and nothing adds it to .gitignore (gitignore maintenance is skipped under managed mode), so every git status in that repo shows an untracked private marker — easy to commit by accident, and noisy for any tooling that checks for a clean tree.

Expected

Exclude the hardening scaffold from import (or place it where sync does not pick it up), and add the ownership marker to the source's ignore rules when hardening/claiming writes it.

Environment

0.51.0.0, hardened filesystem source, managed activation, both PGLite and Postgres.