docs(agents): state the corollary that a local merge-tree of an os-regen path is not GitHub mergeability, and name the sound probe (#15871)

AGENTS.md §11 already says the `merge=os-regen` driver is a LOCAL facility. What
it never stated is the corollary that costs a seat a round trip: a local
`git merge-tree` of a routed path runs the same merge-ort machinery as
`git merge`, so it HONOURS the driver, while GitHub runs none — the two answer
different questions about the same snapshot.

The added bullet names the sound instrument (a throwaway bare clone sharing the
object store, where the driver is genuinely absent, which is GitHub's actual
condition) and refuses the falsified `-c merge.os-regen.driver=` spelling, whose
empty string does not disable the driver but leaves git failing to run it and
reporting a conflict for every routed path — including ones that text-merge
cleanly. Measurements are on PR #15868, which carries the same text in
`scripts/pm/os-regen-merge.sh`'s header.

Part of #15815

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_012zGPuVVX3deAx9LdjK8jCk

Author: claude

AGENTS.md

@@ -650,6 +650,16 @@ Even inside your own worktree, operate defensively:
      (`authorable-surface/`, `json-schema.manifest/`, `api-surface/`) to keep parallel
      spec PRs textually disjoint; every gate reads the whole directory as one set, so
      ratchet semantics are unchanged (`packages/spec/scripts/lib/sharded-artifacts.ts`).
+   - **Corollary — a local `merge-tree` is NOT GitHub's mergeability**, and it has already
+     cost a seat a round trip: a local `git merge-tree` of any `merge=os-regen` path runs
+     the same merge-ort machinery as `git merge` and therefore honours the custom driver,
+     while GitHub runs none, so the two answer different questions about the same snapshot.
+     Probe from a throwaway bare clone that shares the object store and has no driver
+     registered (`git clone --bare --shared . PROBE.git`, then
+     `git --git-dir=PROBE.git merge-tree --write-tree --name-only BASE HEAD`), ⛔ never with
+     `-c merge.os-regen.driver=`, which does not disable the driver but leaves git failing
+     to run it and reporting a conflict for every routed path, including ones that
+     text-merge cleanly.
    - **Registration is per clone** (`pnpm install` → `prepare` →
      `scripts/setup-git-hooks.mjs`); an unregistered clone falls back to git's default
      text merge — older behaviour, not breakage.