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/matrix_state.json

@@ -0,0 +1,4 @@
+{
+  "last_published_line": 0,
+  "since": null
+}