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/repo-hooks.sh

@@ -0,0 +1,17 @@
+#!/usr/bin/env bash
+# Optional repo-specific hooks used by scripts/agent/*.sh
+
+# Called by session-start.sh
+# mosaic_hook_session_start() {
+#   echo "Run repo-specific startup checks"
+# }
+
+# Called by critical.sh
+# mosaic_hook_critical() {
+#   echo "Run repo-specific critical queries"
+# }
+
+# Called by session-end.sh
+# mosaic_hook_session_end() {
+#   echo "Run repo-specific end-of-session checks"
+# }