[probe] exercise the feature path of the Fork gates check - #18

Two scheduled jobs had been red on every run for a fortnight, and both alarms were false. `sync-mirror.yml` masked a `gh label create` failure with `|| true` and then hard-failed on the `gh issue create` that depended on the label. `fork-only-view.yml` failed its first stack invariant because the stack was one rebase behind upstream, returned early, and so silently stopped running the view-drift and standing-PR checks it exists for.

Both are fixed at the root. The tracking issue is now matched by exact title, so no second object has to exist first, and it is the only alarm for an outstanding rebase. `verify_stack` gains `behind_is_notice`: a behind stack is reported as a warning and the remaining invariants are measured against the mirror the stack was built on, so a sync state can no longer disable a different detector.

The rest reduces what every agent session costs. The always-loaded instructions drop from 6056 to 3941 words, with the rationale moved into PROCESS.md where it is read on demand. Review is now tiered: `forkctl track` classifies a diff as A (pufferlib, config, pufferl.py, models.py, setup.py, smoke goldens) or B (everything else), and only Track A owes a fresh-context adversarial review and the simulator suites. Nits are stated as a process bug, not a review comment. The PR template drops from 85 lines and 20 checkboxes to 56 and 7, by deleting everything forkctl check-commit already proves.