feat(inventory): .github is on the train and has an fr-gate caller

[LukasWodka]

The follow-up this file's own note prescribes:

Adding a caller and flipping its entry to required in the same PR therefore fails, because the caller is not on develop yet. Land the caller first, flip the entry after.

.github#137 landed the fr-gate caller and release-train#26 enrolled the repo, so both facts are now true:

entry	was	now
release_train	false	true
fr-gate.yml	exempt: *no_staging_branch_no_hop_to_gate	required

That exemption was accurate until today — .github genuinely had no staging branch, so there was no hop to gate. It has one now (created from main @ ee105cea, so its ancestry already contains prod's), and RFC-BACKEND-1405's open question 3 is answered in favour of branch promotion, which puts this repo on the same develop → staging → main path as everything else.

Verified

• Audit with this change: 2 findings, both rfcs entries that .github#138 fixes.
• Audit without it: 4 — the two above plus .github: release_train mismatch and .github: fr-gate exempt but a caller exists.
• caller-drift-selftest.py: 42 pass, 0 fail.

Sequencing note

This is the fifth cross-repo caller change today that produced a transient red window, and the note quoted above is why this one was unavoidable rather than sloppy: the guard reads the inventory from the checkout but caller state from the live branch, so the assertion cannot be true in the same commit that makes it true.

Bugbot made the same point on #137 when I had bundled the release_train flip there — it correctly noted the PR would fail its own validation. Splitting it out is what let #137 merge green.

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Note

Low Risk
Single-file inventory metadata only; no runtime or auth changes, just aligning the drift guard with merged workflow and branch setup.

Overview
Updates repo-inventory.yml for .github so the caller-drift guard matches what already landed elsewhere: release_train is true (aligned with release-train enrollment), and fr-gate.yml is required instead of exempt under no staging branch.

That exemption is dropped because .github now has a staging branch and an fr-gate-caller.yml on develop; the inventory flip is intentionally sequenced after those changes so the guard (inventory from checkout, caller state from the live branch) stays green.

^{Reviewed by Cursor Bugbot for commit ba40ced. Bugbot is set up for automated code reviews on this repo. Configure here.}