Reduce merge-queue wait to 1 min; add MIGRATION.md entry doc + .vscode README

[DenWin]

Goal

Two things from the repo-concepts review:

1. Queued merge entries waited up to 5 minutes before merging; a solo repo
   gains nothing from batching that long — drop the merge-queue wait to
   1 minute.
2. The template had no single entry document for migrating its concepts into
   a repo not created from it (AGENTS.md's retrofit section was a terse
   list), and the concepts inventory found one documentation gap: AGENTS.md's
   mechanism map links .vscode/README.md, which did not exist.

Scope

• setup/Protect-MainBranch.ps1 — min_entries_to_merge_wait_minutes 5 → 1;
  setup/Protect-MainBranch.Tests.ps1 — new assertion pinning the value
  (test-first).
• MIGRATION.md (new) — entry document: concept→doc map plus migration steps
  in dependency order, linking each per-folder reference doc.
• AGENTS.md — retrofit section now points to MIGRATION.md instead of
  duplicating the steps (one source, no drift).
• .vscode/README.md (new) — fills the missing per-folder doc;
  .gitignore — one-line exception so the README is tracked.

These boundaries are correct because the wait-time change plus its test live
entirely in setup/, and the rest is documentation only.

Risk & rollback

Low. The docs have no runtime behavior. The wait-time change only affects the
ruleset payload the one-time setup script POSTs — it applies to a repo when
setup/Protect-MainBranch.ps1 is (re-)run there with admin + gh; note the
scripts cannot execute from the sandbox this PR was authored in (no gh).
Rollback: revert the commit; an already-applied ruleset can be adjusted in
repo settings → Rules.

Evidence

• Fast-lane Pester run over the whole repo (pwsh 7.6.3, Pester 5.7.1):
  32 passed, 0 failed, including the new
  waits at most 1 minute before merging queued entries assertion.
• Both setup scripts executed in the sandbox to verify their fail-fast guard:
  each exits 1 with GitHub CLI (gh) is not installed. as designed.
• Markdown tables in the new docs are pre-aligned to the MD060: aligned
  rule; full pre-commit suite runs in CI on this PR.

🤖 Generated with Claude Code

https://claude.ai/code/session_01Vi2ifWUY3MXfbcbNFnsS8V

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