feat(mcp): set_pbi_pr + mark_pbi_pr_merged tools voor PBI-PR-gating (#18)

* feat(ST-mhj9f2la): add set_pbi_pr MCP tool

- Add pr_url and pr_merged_at fields to Pbi model in schema
- Implement set_pbi_pr tool: writes pr_url, clears pr_merged_at (idempotent)
- AuthZ via requireWriteAccess + userCanAccessProduct through pbi.product_id
- 10 tests: happy path, not-found, no-access, demo-denied, schema validation
- Update README tools table and bump version to 0.2.0

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

* feat(ST-mhj9f2la): add mark_pbi_pr_merged MCP tool

- Implement mark_pbi_pr_merged: sets pr_merged_at = now() on a PBI
- Requires pr_url to be set; returns error if not (geen gekoppelde PR)
- Idempotent: re-calling overwrites the timestamp
- AuthZ via requireWriteAccess + userCanAccessProduct through pbi.product_id
- 6 tests: happy path, no-pr_url, idempotent, no-access, not-found, demo-denied
- Update README tools table with mark_pbi_pr_merged entry

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

* docs(ST-mhj9f2la): expand README with set_pbi_pr + mark_pbi_pr_merged docs

Add full signature/input/output/error documentation sections for both
new tools, following the verify_task_against_plan pattern.
Version already bumped to 0.2.0 in earlier commit.
Tag + MCP_GIT_REF pin in scrum4me-docker to be done by maintainer after merge.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

---------

Co-authored-by: Claude Sonnet 4.6 <noreply@anthropic.com>

prisma/schema.prisma

@@ -161,8 +161,10 @@ model Pbi {
   description String?
   priority    Int
   sort_order  Float
-  status      PbiStatus @default(READY)
-  created_at  DateTime  @default(now())
+  status       PbiStatus @default(READY)
+  pr_url       String?
+  pr_merged_at DateTime?
+  created_at   DateTime  @default(now())
   updated_at  DateTime  @updatedAt
   stories     Story[]