Add iOS debugging guidance and layer architecture docs to prevent tunnel vision failures

[Copilot]

Context

PR #33353 revealed an agent failure mode: 11 attempts patching ShellSectionRootRenderer (where stack trace pointed), but the fix was removing that duplicate and enhancing PageViewController (Core layer). Agent never searched for duplicates or understood layer separation.

Changes

New Documentation

.github/instructions/ios-debugging.instructions.md - iOS platform debugging patterns

• Search method patterns across codebase, not just stack trace location
• Detect duplicate implementations with grep -r "MethodName" src/
• "Remove vs Patch" decision tree for obsolete code
• Layer architecture (Core vs Controls vs Compatibility)

.github/architecture/core-vs-controls.md - Layer responsibility quick reference

• Decision matrix: Core (all pages) vs Controls (specific controls)
• Common mistakes (Core functionality in Controls, duplication)
• iOS lifecycle method placement guide

Enhanced Workflow

.github/skills/try-fix/SKILL.md - Added Step 3: Search for Duplicate Implementations

# Before patching, check for duplicates across layers
grep -r "override.*TraitCollectionDidChange" src/
find src/Core src/Controls -name "*.cs" -exec grep -l "MethodName" {} \;

Code Documentation

Added layer responsibility comments to:

• PageViewController.cs - Documents Core layer applies to ALL pages
• ShellSectionRootRenderer.cs - Explains why TraitCollectionDidChange was removed (PR [iOS] Fix ObjectDisposedException in TraitCollectionDidChange on window disposal #33353)

Integration

.github/copilot-instructions.md - Added "Understanding Core vs Controls Layers" section with references

Impact

Future agents facing similar iOS crashes will:

1. Search for method patterns across layers (not just named file)
2. Detect duplicates between Core/Controls implementations
3. Consider removing obsolete code vs patching
4. Understand layer separation (Core = all pages, Controls = specific)

Directly addresses the tunnel vision pattern where agents trust stack traces without verifying if functionality is duplicated elsewhere.

Original prompt

Please review the following PR #33353 and recommend areas where we could have improved our repository so the pr agent can be more successul in the future

Custom agent used: learn-from-pr
Analyzes completed PRs for lessons learned, then applies improvements to instruction files, skills, and documentation.

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