docs(handoff): the index write landed, the gate went terminal, and the finish sequence has a non-optional re-verify step

[ZacxDev]

Fourth and final handoff update for the cairn-oss-multi-instance arc (after #1492, #1530, #1546). No code changes.

What changed since #1546 merged minutes ago:

• chore(cairn): consolidate onto the pinned client — delete the five forked reader modules #1508's gate went terminal: tekton/devrc-pytests fail (main's red), nodetests pass, cairn-client-runs pass. scripts/tests — the target this PR changes — is 14038 passed / 0 failed.
• The subsystem-index write is DONE — devrc/cairn, revision 389daf17211c0dcf, validated at the writer (32 of 32 parse, 0 malformed, 0 dropped lines, 0 out-of-reach markers). Earlier revisions of this doc recorded it as an outstanding step; it is no longer.
• Claim cairn-oss-multi-instance-20 is still held by nobody pursuing it — recorded, because an unreleased claim is the only thing that blocks another session from an item.
• The finish sequence is now written out with step 2 marked non-optional: re-run the two test_guard_core.py tests at main before trusting fix(guard-core): main is RED — two handoff docs mention a wide tmux kill and were classified in NEITHER ledger #1543's merge. That assumption was already wrong once today — after fix(guard tests): the wide-kill mention scanner matched inside skill-session — red on main #1534 merged, main was red again on a different file within minutes.
• Publishing a store-api image from the merged tree is listed as step 5. The build path was rewired but no image was pushed, so the live pod keeps running the old forked reader and verify-byte-identity.sh is expected to mismatch until then.

One new gotcha worth the line: subsystem_touch.py --session is structurally blind to a dispatch-hub session — from a session whose cwd was another repo it refused with "transcript cwd does not match … 0 paths under this repo", and the tool's own advice is not to fall back to the git window. With PRs in hand, --pr 1508,1546 resolved 48 paths and nominated real entries. So an index write that is impossible early in such a session becomes possible the moment a PR exists — re-run it rather than recording the step as declined.