Add architecture guidance and capability-aware escalation to skills

Add design-for-isolation and working-in-existing-codebases guidance to
brainstorming. Add file size awareness and escalation prompts to SDD
implementer and code quality reviewer. Writing-plans gets architecture
section sizing guidance. Spec and plan reviewers get architecture and
file size checks.

skills/brainstorming/spec-document-reviewer-prompt.md

@@ -23,6 +23,7 @@ Task tool (general-purpose):
     | Consistency | Internal contradictions, conflicting requirements |
     | Clarity | Ambiguous requirements |
     | YAGNI | Unrequested features, over-engineering |
+    | Architecture | Units with clear boundaries, well-defined interfaces, independently understandable and testable |
 
     ## CRITICAL
 
@@ -30,6 +31,7 @@ Task tool (general-purpose):
     - Any TODO markers or placeholder text
     - Sections saying "to be defined later" or "will spec when X is done"
     - Sections noticeably less detailed than others
+    - Units that lack clear boundaries or interfaces — can you understand what each unit does without reading its internals?
 
     ## Output Format