docs: expand anti-patterns with private state access guidance

[colombod]

Summary

Expands the anti-patterns section in DESIGN_PHILOSOPHY.md with explicit guidance about accessing private state via getattr(obj, '_private', ...).

Motivation

This PR was motivated by microsoft/amplifier-app-cli#70 which demonstrates a "fix" that reaches into private internal state instead of using public APIs:

# The anti-pattern from PR #70
bundle_resolver = getattr(resolver, "_bundle", resolver)  # Unwrap layer 1
activator = getattr(bundle_resolver, "_activator", None)   # Unwrap layer 2

The comment "Pattern from session_spawner.py" shows this anti-pattern is spreading across the codebase, making it important to document why this is wrong.

What's Added

New subsection "Accessing Private State (Critical Anti-Pattern)" under Module Development anti-patterns:

• Why this pattern is dangerous (changes without warning, multiplied fragility)
• Code example showing wrong vs right approach
• Guidance on when internal access seems needed
• Litmus test for detecting protocol violations

Design Philosophy Alignment

This aligns with existing principles:

• "Depending on kernel internals (use protocols only)" - now with explicit examples
• "Mechanism, Not Policy" - private state is implementation detail, not contract
• "Bricks & Studs" - interfaces (studs) should be used, not internal brick structure

Related

• Companion PR: docs: add encapsulation boundary rules (Principle 7) amplifier-bundle-python-dev#5 (adds to PYTHON_BEST_PRACTICES.md)
• Motivating issue: fix: unwrap AppModuleResolver to find activator in self-healing amplifier-app-cli#70

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🤖 Generated with Amplifier

Co-Authored-By: Amplifier 240397093+microsoft-amplifier@users.noreply.github.com

[colombod]

Closing in favor of adding this to IMPLEMENTATION_PHILOSOPHY.md in amplifier-foundation instead - that's the right place for development-time guidance.