PBI-9 + PBI-47: worktree foundation, product-worktrees, P0 fixes, PAUSED flow

Adds two interlocking PBIs:

PBI-9 — Worktree foundation + persistent product-worktrees for idea-jobs
  - src/git/worktree-paths.ts: centralised root + skip-set + lock-path helpers
  - src/git/file-lock.ts: proper-lockfile wrapper, deadlock-safe ordered acquire
  - src/git/product-worktree.ts: detached-HEAD worktree per product, .scratch/
    excluded via git rev-parse --git-path (handles linked .git file)
  - src/git/job-locks.ts: setupProductWorktrees + releaseLocksOnTerminal
  - wait-for-job.ts: idea-branch wires product-worktrees for IDEA_GRILL/MAKE_PLAN
  - update-job-status.ts + pbi-cascade.ts + stale-reset: release on all four
    server-side terminal transitions (DONE/FAILED/CANCELLED/stale)
  - cleanup-my-worktrees: skip _products/ + *.lock
  - README: worktrees section with single-host invariant + advisory-lock path

PBI-47 — Sprint-flow P0 corrections + PAUSED flow with rich pause_context
  - prisma schema: ClaudeJob.{base_sha,head_sha} + SprintRun.pause_context
  - tryClaimJob captures base_sha; prepareDoneUpdate captures head_sha
  - verify-task-against-plan diffs vs base_sha (no more origin/main fallback);
    rejects with MISSING_BASE_SHA when null — fixes per-task verify-scope P0
  - pr.ts: createPullRequest enableAutoMerge default false; new
    enableAutoMergeOnPr with --match-head-commit guard + 5-category typed
    EnableAutoMergeResult — fixes STORY auto-merge timing P0
  - src/flow/{effects,worktree-lease,pr-flow,sprint-run}.ts: pure transition
    modules + idempotent declarative effects executor
  - update-job-status: STORY auto-merge fires only on the last task of the
    story (story.status === DONE), with head_sha as merge guard; MERGE_CONFLICT
    routes to sprint-run flow which produces CREATE_CLAUDE_QUESTION +
    SET_SPRINT_RUN_STATUS effects with rich pause_context

Tests: 31 test files, 242 passing. Pure-transition tests cover STORY 3-tasks
auto-merge timing, SPRINT draft→ready, MERGE_CONFLICT pause/resume, file-lock
deadlock prevention, worktree-lease lifecycle, delete-only verify (ALIGNED),
per-job verify scope (base_sha isolation), 5-category auto-merge errors.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

package.json

@@ -32,7 +32,9 @@
     "@modelcontextprotocol/sdk": "^1.29.0",
     "@prisma/adapter-pg": "^7.8.0",
     "@prisma/client": "^7.8.0",
+    "@types/proper-lockfile": "^4.1.4",
     "pg": "^8.13.1",
+    "proper-lockfile": "^4.1.2",
     "yaml": "^2.8.4",
     "zod": "^4.0.0"
   },