Landed-note SHA citations die when a branch is rebased before its gate

Collapsing an Implementation section to "shipped at <sha>`" notes is a gate obligation, but the SHAs an implementer has in hand at that moment are their pre-rebase ones, and trunk-based development guarantees the rebase. The grammar item’s collapsed section cited `71f27d0, c97a8de and f4dbbe3; none resolve, because the branch was rebased when trunk moved and the commits became b48b0f8, 7eb474f and dd77f66. The citation is worthless to a later reader in exactly the case it exists for. This is generic to the workflow rather than to one item: any item that writes its own landing SHAs before its final trunk sync records numbers that will not survive.

Cheapest fix is probably guidance rather than machinery, since the spec file is deleted at the gate anyway and the durable record is roadmap/changelog.md: say in roadmap/workflow.adoc that landing SHAs are the reviewer’s to write at the Done gate, from the post-sync history, and that a collapsed Implementation section should name commits by subject line rather than by SHA. A mechanical check is possible (git cat-file -e every [0-9a-f]{7,} in roadmap/*.md) but would false-positive on the changelog’s own historical SHAs from before any repository rewrite, so weigh it against the guidance option rather than assuming the gate.

Filed at the grammar item’s In Review → Done gate, where the reviewer wrote the correct SHAs into the changelog by hand.