fix(node-sync): materialize the git identity into the context mount, not the overlay home

[gbasin]

What

Agent commits still landed as Centaur AI after #546 — with the identity file sitting on disk, readable from the node. This moves the [include] target from the agent's overlay home onto the context mount (~/context/.atrium-git-identity).

Why the overlay home can never work

The include mechanism poisons itself, and the ordering is not a race — it's guaranteed:

entrypoint.sh runs git config at pod creation. git reads /opt/centaur/gitconfig, resolves the [include] target, gets ENOENT, and the kernel caches a negative dentry — minutes before a claim exists to say who the user is. By the time node-sync writes the identity, the lookup has already missed. The only question is whether that negative entry gets invalidated.

Overlay mount instances keep independent dentry trees, so a node-side create never invalidates the pod's entry — neither writing into upper (shipped, failed) nor through merged (#546, failed). The forensic signature is readdir listing the file while lookup returns ENOENT.

A bind mount of a single ext4 superblock shares one dentry tree, so the create is observed even after a failed lookup. ~/context is exactly that (hostPath /var/lib/centaur/atrium/<session>, readOnly on the pod side — the agent can read its identity but can't clobber it).

Measured on a live prod pod, both ways, before this was written.

Verification

Negative control in a privileged container with a real-fs upperdir — proves the test discriminates rather than passing vacuously:

topology	pod lookup after node-side create
overlay, two instances, shared upper (#546)	BLIND — readdir shows it, lookup ENOENT
overlay, node writes straight to upper	BLIND
bind mount, one ext4 superblock (this PR)	SEES IT

Privileged Docker gate green: cargo fmt --check, cargo clippy --all-targets -D warnings, full cargo test (27 tests) including the new git_identity_visibility running as root (not skipped).

Prod confirmed on CENTAUR_SANDBOX_OVERLAY_FLAT_HOME=true, so ~/context is the context mount path.

Notes

• The new test asserts the ordering that matters: mount, MISS, write, hit. The fix(node-sync): write the git identity through the merged view, not the overlay upper #546 test wrote before looking up, so it passed on the broken design — that's how this shipped twice.
• CI's cargo test lane is unprivileged and reports this file as "ok" while skipping every case; the privileged step (renamed here) is what actually runs it.
• Failures still never gate the mount — an absent identity degrades to the baked Centaur AI, i.e. pre-existing behavior.