docs(architecture): archive completed pql migration plan

The migration shipped (Phases 0-5 in PR #154; phases-as-hierarchy + Phase-6 SQLite
retirement in PR #155), so the plan is done. Moved pql-migration.md from the repo root
to docs/architecture/, flipped status approved -> completed, and added a header pointing
to the current-usage docs (CLAUDE.md, ticket-cli.md, governance/README.md) and the
tooling/pql-migrate/ transforms. Kept as a why/how record of the move off the binary
SQLite DB. pql-requirements.md stays at root — still-open feedback for the pql team.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

docs/architecture/pql-migration.md

@@ -2,13 +2,20 @@
 title: "pql Migration Plan — decisions + ticketing"
 description: "Migrate the SQLite-backed ticket system and markdown decisions sync onto pql (markdown-vault indexer + planning tool)"
 type: plan
-status: approved
+status: completed
 created: 2026-06-06
 updated: 2026-06-06
 decision_refs: []
 tags: [migration, tooling, pql, decisions, ticketing]
 ---
 
+> **COMPLETED & ARCHIVED (2026-06-06).** This plan shipped: Phases 0–5 in PR #154,
+> the phases-as-hierarchy refactor + Phase-6 SQLite retirement in PR #155. It is kept
+> as a historical record of *why and how* the planning system moved off the binary
+> SQLite DB onto pql. **Current usage** is in `CLAUDE.md`, `.claude/rules/ticket-cli.md`,
+> and `governance/README.md`; the one-shot transforms live in `tooling/pql-migrate/`.
+> Open feedback for the pql team is in the root `pql-requirements.md`.
+
 # pql Migration Plan — decisions + ticketing
 
 > **Status:** approved, not yet executed. All work happens on the `pql-migration`