Spec Kitty

Mission Run: Anchor Gate D's dead-path docs regex

Stop the dead-path docs gate from flagging any nested .../src/ path as a top-level src/ reference (#6096).

Gate D enforces that live docs name no retired top-level src/charter/offering/<kind>/built-in home. Its git grep regex has no left boundary, so a longer path whose tail is that shape is wrongly flagged. This mission tightens the left boundary only, with a red-first non-vacuity regression test, so the gate stays precise without loosening what it forbids.