feat(release): recursion guard for the health gate; run-task drift warning; M20 packages/* decision recorded (#39)

- release.sh health gate runs with MOSAIC_ENSURE_SKIP=1: the gated task run
  cannot re-enter release self-determination
- run-task.sh warns on release drift instead of silently using a stale image
- ROADMAP: M20 decision recorded (packages/* monorepo at usurpation,
  continuity-first); restructure sequenced as M20 phase 1

Closes #39

skills/ms-file-write/SKILL.md

@@ -0,0 +1,18 @@
+---
+name: ms-file-write
+description: Use this skill when writing any file.
+disable-model-invocation: false
+---
+
+# ms-file-write
+
+Write files so the write is provable and reversible.
+
+## Rules
+
+1. Write to a temp file and rename for atomic replacement of an existing file.
+2. Verify the write: re-read or checksum before claiming success.
+3. Never overwrite a file that carries identity or history (seat records,
+   run records, logs). They are append-only or write-once for a reason.
+4. No secrets in written files. Ever.
+5. Match the file's existing style. Do not reformat regions you did not touch.