chore: number specs and plans (4-digit per-directory counter)

Switch naming.policy from flat to stable_per_directory_4digit and list
docs/specs + docs/plans as counter_dirs, so cycle artifacts sort and
reference by a stable 4-digit slot (0001-<slug>.md).

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

.claude/dev-cycle-profile.yml

@@ -9,8 +9,8 @@ paths:
   code_roots: [crates]
 
 naming:
-  counter_dirs: []
-  policy: flat
+  counter_dirs: [docs/specs, docs/plans]
+  policy: stable_per_directory_4digit
   slug_separator: "-"
 
 commands: