chore: planning gate — milestones, issues, and task breakdown

Break PRD into 8 milestones (Phase 0–7) with 59 issues on Gitea.
Populate TASKS.md, update mission manifest, initialize scratchpad.
Repo created at git.mosaicstack.dev/mosaic/mosaic-stack.

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

.mosaic/orchestrator/session.lock

@@ -0,0 +1,8 @@
+{
+  "session_id": "claude-20260312-194506-357136",
+  "runtime": "claude",
+  "pid": 357136,
+  "started_at": "2026-03-13T00:45:06Z",
+  "project_path": "/home/jwoltje/src/mosaic-mono-v1",
+  "milestone_id": ""
+}