feat(mcp): add multi-user sharing ACL guard to _INSTRUCTIONS

The ACL constraint (scope every read/mutation by owner + shares via
services/access.py) is a security-correctness invariant that should
always be loaded, and it's FabledScribe-specific — so it belongs in
Scribe's own contained _INSTRUCTIONS, not the cross-project FabledSword
rulebook. The redundant rulebook rule will be retired once this ships
to prod.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/fabledassistant/mcp/server.py

@@ -122,6 +122,13 @@ X process" or otherwise references a saved process, call list_processes() /
 get_process(name) and follow the returned prompt verbatim, including any
 "clarify first" steps it contains. Author a new one with create_process(title,
 body); edit with update_process.
+
+When you are developing Scribe itself (not just using it as a data store),
+honor the multi-user sharing ACL: every read or mutation of user data must
+scope by owner + direct shares + group shares through services/access.py
+(can_read_* / can_write_* / can_admin_*) — never assume a single operator. An
+unscoped query (a fetch-by-id with no ownership check) is a cross-user data
+leak; "works for one user" is not done.
 """