fix(#3699): report why a derived frontmatter key was not written, and repair a missing body source (#3846)

* test(#3699): failing-first coverage for derived-key reporting and the case-D fallback

* fix(#3699): report why a derived frontmatter key was not written, and repair a missing body source

* fix(#3699): scope session-field writes to ## Session so an archived line cannot absorb the update

* fix(#3699): resolve the session writer from body labels only, so a frontmatter key never writes the body

* chore(#3699): backfill changeset PR number

---------

Co-authored-by: sim <sim@local>

docs/CLI-TOOLS.md

@@ -45,7 +45,8 @@ node gsd-tools.cjs state load
 # Output STATE.md frontmatter as JSON
 node gsd-tools.cjs state json
 
-# Update a single field
+# Update a single field. Frontmatter keys are projections of body fields —
+# write the body field (see COMMANDS.md#state-update-field-value).
 node gsd-tools.cjs state update <field> <value>
 
 # Get STATE.md content or a specific section