feat(pi): edpa_pi_create tool + /edpa:create-pi command & skill (2.2.0)

[jurby]

What

Adds a first-class way to create the PI-level metadata file
.edpa/iterations/<PI-YYYY-N>.yaml (top-level pi: block). Until now only
per-iteration files had tooling (edpa_iteration_create); the PI parent had to
be hand-written — and the loader globs *.yaml only, so a PI-2026-1.yml was
silently ignored.

Architecture — script-first (single source of behavior)

create_pi.py owns the logic; every interface delegates to it:

• create_pi.py — importable create_pi() core (validate PI-level id,
  refuse overwrite, atomic write of the pi: block) + CLI (validate +
  auto-commit). Self-contained; no dependency on the MCP layer.
• edpa_pi_create MCP tool — thin delegate that imports create_pi()
  (write only, no commit, like the other write tools).
• /edpa:create-pi command + edpa:create-pi skill — shell out to the
  CLI (like /edpa:capacity → capacity_override.py).

Does not scaffold child iterations (those are edpa_iteration_create).
Rejects iteration-level ids (PI-YYYY-N.M) and duplicates.

Release

• bump 2.1.9 → 2.2.0 (plugin.json, web/package.json + lock, tmpl methodology)
• CHANGELOG.md entry; tool surface now 7 read + 8 write

Tests / docs

• tests/test_create_pi.py (core + CLI + loader round-trip); edpa_pi_create
  added to the MCP write / idempotency suites and the advertised-tool assertions
• docs/mcp.md (write-tools note + corrected "read-only" claim),
  docs/playbook.md §1.5, docs/RUNBOOK.md, plugin/README.md
• pytest tests/ → 600 passed

Built in an isolated worktree off origin/main (2.1.9). Note: a parallel
branch chore/drop-dead-cadence is in flight locally toward 2.1.10 — review
release sequencing before merging/deploying.

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