refactor: fold ak x aqe-store and ak x skills into maintain

[pacphi]

Why

Ledger decision G12.

In scope

The stray-store merge becomes a maintain action with an undo receipt (never a sync step, B5-D2); the skills plan becomes a read-only finding.

Out of scope

New maintenance actions.

Acceptance criteria

• The merge refuses while an AQE writer is open
• maintain undo reverses a merge

Dependencies

Blocked by: #360

Blocks: #387

Plan

• Release: 4.0.0-beta.3
• Size: size:M · Workstream: ws:project-scope
• Epic: feat: project-scope P5, sync, status, uninstall and the command folds (epic) #300
• Card: P5-07 in the card inventory
• Sources: Supersession ledger (docs(plans): v4 GA master plan, supersession ledger and v5 registers #290) G12

Verification

• node scripts/run-tests.mjs focus (new maintain tests)