[fm-refit] Expand sync maintenance into refit - #64

Validation notice

Validation was not run by the captain's explicit choice for this direct-PR task.
The required PR must be raised via no-mistakes provenance check is therefore expected to be red; the substance of this change is not expected to be red for that reason.
No no-mistakes pipeline was run.

What changed

• Renamed the internal syncfirstmate skill to refit, retaining the old invocation only as a documented alias.
• Expanded the skill into four legs: CURRENCY, FIT, MEMORY through /stow, and INTEGRITY.
• Updated the firstmate contract, documentation inventory, helper ownership comments, and stow contract test.
• Kept /updatefirstmate separate as the faster propagation operation and preserved the upstream check/full-sync safety rules.

Local checks

• bash tests/fm-stow-contract.test.sh
• bash tests/fm-documentation-audiences.test.sh
• bash tests/fm-external-tooling-check.test.sh
• bash bin/fm-doc-audience-check.sh
• bash bin/fm-lint.sh
• git diff --check

Delivery

Direct-PR was explicitly selected by the captain.
The PR must not be merged without the captain's explicit word.