kit hardening batch: promote re-run branch collision, Unavailable dedup, misc

[amis92]

Fast-follow batch from the M3 final review triage:

• repeated /muster promote on the same issue fails on git push (branch muster/promote-issue-N already exists) with no user-facing explanation — force-push or delete-and-recreate
• BlastRadius.ClassifyMulti can list an engine twice under Unavailable (probe flake between base/head sides) — HashSet dedup
• NR list parser: negative amount passes through silently as a negative count — route to Unmapped
• force-skip Unmapped note should include the force id (spec §4: reply 'names the missing ids')
• entrypoint.sh usage error line doesn't mention the new positionals
• root-owned outputs via docker run on self-hosted runners — document or add --user
• docs: mention snapshot carry-forward in the report-mode snippet; soften the preservation-note wording (also shown when the reporter edited the roster out)

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https://claude.ai/code/session_01KcYK8hmTmXi8eLWpg9LGKJ