Migrate DSPy integration from Snakepit to DSPex (v0.2.1)

Complete architectural decoupling following clean separation of concerns.

Python Changes:
- Add dspy_variable_integration.py (469 LOC extracted from Snakepit)
- VariableAwarePredict, ChainOfThought, ReAct, ProgramOfThought
- ModuleVariableResolver and helper functions
- Update dspex_adapters __init__.py with new module

Elixir API Migration (9 modules):
- Migrate DSPex.Config to use check_dspy tool
- Migrate DSPex.LM to use configure_lm tool
- Migrate DSPex.Settings, Examples, Assertions
- Migrate Retry, ProgramOfThought, MultiChainComparison modules
- Migrate ReAct module (marked not_implemented - needs tool registration)
- Remove redundant DSPex.Python.Bridge module
- All using Snakepit.execute_in_session/3 instead of removed Python.call/3

Dependency Updates:
- Update Snakepit dependency: 0.4.2 → 0.4.3
- Snakepit v0.4.3 deprecates DSPy integration in favor of DSPex

Testing:
- All 82 tests passing
- Zero Snakepit.Python.call/3 references remaining
- Clean compilation (only minor unused variable warnings)

Documentation:
- Added comprehensive architecture review docs
- Updated CHANGELOG for v0.2.1
- Bumped version to 0.2.1 in mix.exs

See: docs/architecture_review_20251007/ for complete analysis

Related:
- Snakepit v0.4.3 deprecation
- Architecture Decision Record (ADR-001)

🤖 Generated with Claude Code

Co-Authored-By: Claude <[email protected]>

Update mermaid configuration to match snakepit

- Added separate head and body tag functions for mermaid setup
- Included custom theme variables for consistent styling
- Improved dark mode support with proper theme switching
- Changed startOnLoad to true for automatic diagram rendering

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <[email protected]>

Release v0.1.2

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