feat: add architecture guidance and capability-aware escalation to skills

Brainstorming: design-for-isolation guidance and brownfield codebase awareness
Writing-plans: file structure section requiring decomposition before task definition
Implementer prompt: code organization awareness, structured escalation protocol
  (DONE/DONE_WITH_CONCERNS/BLOCKED/NEEDS_CONTEXT), explicit permission to stop
Subagent-driven-development: provider-agnostic model selection tiers, escalation handling

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 work best with files you can read in a single pass. Large files cause silent truncation, context degradation, and unreliable edits. 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):**