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/writing-plans/SKILL.md

@@ -18,6 +18,17 @@ Assume they are a skilled developer, but know almost nothing about our toolset o
 **Save plans to:** `docs/superpowers/plans/YYYY-MM-DD-<feature-name>.md`
 - (User preferences for plan location override this default)
 
+## File Structure
+
+Before defining tasks, map out which files will be created or modified and what each one is responsible for. This is where decomposition decisions get locked in.
+
+- Design units with clear boundaries and well-defined interfaces. Each file should have one clear responsibility.
+- You reason best about code you can hold in context at once, and your edits are more reliable when files are focused. Prefer smaller, focused files over large ones that do too much.
+- Files that change together should live together. Split by responsibility, not by technical layer.
+- In existing codebases, follow established patterns. If the codebase uses large files, don't unilaterally restructure - but if a file you're modifying has grown unwieldy, including a split in the plan is reasonable.
+
+This structure informs the task decomposition. Each task should produce self-contained changes that make sense independently.
+
 ## Bite-Sized Task Granularity
 
 **Each step is one action (2-5 minutes):**

skills/writing-plans/plan-document-reviewer-prompt.md

@@ -22,6 +22,8 @@ Task tool (general-purpose):
     | Completeness | TODOs, placeholders, incomplete tasks, missing steps |
     | Spec Alignment | Chunk covers relevant spec requirements, no scope creep |
     | Task Decomposition | Tasks atomic, clear boundaries, steps actionable |
+    | File Structure | Files have clear single responsibilities, split by responsibility not layer |
+    | File Size | Would any new or modified file likely grow large enough to be hard to reason about as a whole? |
     | Task Syntax | Checkbox syntax (`- [ ]`) on steps for tracking |
     | Chunk Size | Each chunk under 1000 lines |
 
@@ -32,6 +34,7 @@ Task tool (general-purpose):
     - Steps that say "similar to X" without actual content
     - Incomplete task definitions
     - Missing verification steps or expected outputs
+    - Files planned to hold multiple responsibilities or likely to grow unwieldy
 
     ## Output Format