[security] Encryption key not gitignored; OKF bundle repo has no .gitignore

[stancsz]

Severity: High — surfaced in the architect review (2026-06-20).

R13 selective encryption (iter-22) protects private Concepts, but the surrounding secret hygiene is incomplete:

• The encryption key ($SECONDBRAIN_KEY_FILE / ~/.secondbrain/secret.key) is not matched by any .gitignore rule.
• sync.ensure_repo() creates the OKF bundle repo with git init only — the bundle repo gets no .gitignore at all (scripts/sync.py:28).

For a privacy product this is the one mistake you cannot make: if a key file or brain.db ever lands inside the bundle directory, the next sync commits and pushes it.

Acceptance criteria

• The repo .gitignore (and a generated bundle-repo .gitignore) exclude *.key, secret.key, brain.db*, and any conflict/trash artifacts that must not be pushed.
• sync.ensure_repo() writes a .gitignore into the bundle repo on init.
• A verifier asserts: a key file placed in the bundle dir is never staged/committed by sync.