Relocate migrations/ mission-closeout evidence out of the runbook zone

[stijn-dejongh]

Why

The Common-Docs section audit (docs/plans/engineering-notes/common-docs-section-audit.md)
found docs/migrations/ — the durable migration-runbook zone — carrying one stray dated
mission-closeout record: teamspace-mission-state-920-closeout.md self-describes as
"Closeout evidence for TeamSpace mission-state issue #920, generated from clean workspace
spec-kitty-20260510-…" — a dated mission closeout report (concern bucket (c)), not a
reusable migration runbook.

What

• Move migrations/teamspace-mission-state-920-closeout.md → plans/engineering-notes/.
  Borderline — the audit flags it carries a migration-note banner; verify it is not the
  sole runbook for that migration path before moving (if it doubles as the only guidance,
  split: keep a thin runbook stub in migrations/, move the dated closeout narrative to
  plans/engineering-notes/).
• Sweep migrations/ for any other dated closeout residue beyond this one file.
• Normalize the odd 06_migration_and_shim_rules.md filename residue (numeric prefix is
  naming residue, not a placement defect — rename in place, no move).
• Confirm teamspace-mission-state-repair.md and 2-1-main-cutover-checklist.md stay put
  (durable repair runbook / checklist — correctly placed per the audit).

IA-mechanics obligations

• Redirect-map entry for the moved path (scripts/docs/redirect_map.yaml)
• Relative-link fix (relative_link_fixer --check)
• Page-inventory regen (docs/development/3-2-page-inventory.yaml lockfile +
  check_docs_freshness --ci)
• related: frontmatter edges on the moved file + referrers
• Terminology guard (tests/architectural/test_no_legacy_terminology.py)

Do not double-pick — in-flight mission already implements this

An in-flight mission docs-structural-sanity-01KY53KJ (branch
docs/common-docs-section-audit on the stijn-dejongh fork) already implements this exact
finding — specced + planned + tasked, WP01/03/04 content done, currently parked on the
#2841 coord-tooling blocker. Check that mission before starting fresh work on this ticket.

Sizing

Size S (per audit).

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Source: docs/plans/engineering-notes/common-docs-section-audit.md (candidate 4).