docs(mosaic): declare repo structure v2 (T51 WP2a)

Replaces the v1 two-key declaration with the schema_version 2 form per
mosaic-brain docs/plans/2026-08-23_repo-structure-declaration.md section
1.3 (spec of record @ brain main 515bcbab): flow, canonical_remote,
canonical_clone, worktree_root, and transitional worktree_policy
(orchestrator-precreated until the J3/#1174 amendment). Validated by the
wave-1 validator: OK in managed mode with launch-equivalent anchors;
fail-closed on absent anchors per spec 1.2a interim; display mode warns
and passes. Docs-only, exactly one file.

.mosaic/repo.json

@@ -1,4 +1,11 @@
 {
+  "schema_version": 2,
   "integration_trunk": "next",
-  "release_branch": "main"
+  "release_branch": "main",
+  "flow": "trunk-release",
+  "canonical_remote": "https://git.mosaicstack.dev/mosaicstack/stack",
+  "canonical_clone": "host:/src/mosaic-stack",
+  "worktree_root": "host:/src/mosaic-stack-worktrees",
+  "worktree_policy": "orchestrator-precreated",
+  "notes": "next=development/integration; main=production release. Never branch work off main. worktree_policy is TRANSITIONAL: the wrapper worktree consumer is BLOCKED on the J3/#1174 amendment (checked roots + capacity guard); pre-creation is the interim orchestration choice, not closed policy — it becomes a timing choice only after the wrapper can validate this root."
 }