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/subagent-driven-development/implementer-prompt.md

@@ -41,6 +41,36 @@ Task tool (general-purpose):
     **While you work:** If you encounter something unexpected or unclear, **ask questions**.
     It's always OK to pause and clarify. Don't guess or make assumptions.
 
+    ## Code Organization
+
+    You reason best about code you can hold in context at once, and your edits are more
+    reliable when files are focused. Keep this in mind:
+    - Follow the file structure defined in the plan
+    - Each file should have one clear responsibility with a well-defined interface
+    - If a file you're creating is growing beyond the plan's intent, consider whether
+      it should be split into smaller units with clear boundaries
+    - If an existing file you're modifying is already large or tangled, work carefully
+      and note it as a concern in your report
+    - In existing codebases, follow established patterns. Improve code you're touching
+      the way a good developer would, but don't restructure things outside your task.
+
+    ## When You're in Over Your Head
+
+    It is always OK to stop and say "this is too hard for me." Bad work is worse than
+    no work. You will not be penalized for escalating.
+
+    **STOP and escalate when:**
+    - The task requires architectural decisions with multiple valid approaches
+    - You need to understand code beyond what was provided and can't find clarity
+    - You feel uncertain about whether your approach is correct
+    - The task involves restructuring existing code in ways the plan didn't anticipate
+    - You've been reading file after file trying to understand the system without progress
+
+    **How to escalate:** Report back with status BLOCKED or NEEDS_CONTEXT. Describe
+    specifically what you're stuck on, what you've tried, and what kind of help you need.
+    The controller can provide more context, re-dispatch with a more capable model,
+    or break the task into smaller pieces.
+
     ## Before Reporting Back: Self-Review
 
     Review your work with fresh eyes. Ask yourself:
@@ -70,9 +100,14 @@ Task tool (general-purpose):
     ## Report Format
 
     When done, report:
-    - What you implemented
+    - **Status:** DONE | DONE_WITH_CONCERNS | BLOCKED | NEEDS_CONTEXT
+    - What you implemented (or what you attempted, if blocked)
     - What you tested and test results
     - Files changed
     - Self-review findings (if any)
     - Any issues or concerns
+
+    Use DONE_WITH_CONCERNS if you completed the work but have doubts about correctness.
+    Use BLOCKED if you cannot complete the task. Use NEEDS_CONTEXT if you need
+    information that wasn't provided. Never silently produce work you're unsure about.
 ```