feat: per-slot model env overrides and model validation

[mimran-khan]

Summary

• Add SKILLSPECTOR_MODEL_{SLOT} environment variables for per-slot model selection (e.g., SKILLSPECTOR_MODEL_META_ANALYZER=gpt-4o, SKILLSPECTOR_MODEL_SEMANTIC_DEVELOPER_INTENT=gpt-4o-mini), enabling cost optimization and independent model failure workarounds
• Add model validation at startup that warns when configured models are not found in the provider's model_registry.yaml, with opt-in strict mode (SKILLSPECTOR_STRICT_MODEL_VALIDATION=true) that fails fast instead of silently using fallback token budgets

Per-slot model precedence

SKILLSPECTOR_MODEL_{SLOT} > provider SLOT_DEFAULTS > SKILLSPECTOR_MODEL > provider DEFAULT_MODEL

Changes

• src/skillspector/constants.py — new _resolve_slot_model() function with per-slot env var support; new _validate_model_config() with warning + strict mode
• tests/unit/test_constants.py — 11 tests covering env var precedence, fallback chain, warning emission, and strict mode

Closes #176, closes #177

Test plan

• All 11 new tests pass (pytest tests/unit/test_constants.py -v)
• All 370 existing unit tests pass (no regressions)
• Per-slot env var overrides provider default
• Per-slot env var overrides global SKILLSPECTOR_MODEL
• Whitespace-only env var treated as unset
• Unknown model logs warning with model name and fallback context length
• Known model produces no warning
• Strict mode raises ValueError on unknown models
• Strict mode passes with known models

[rng1995]

[Automated SkillSpector Review]

Approved.

Per-slot SKILLSPECTOR_MODEL_{SLOT} overrides with a clear precedence chain, plus startup model validation (warn, or fail-fast under SKILLSPECTOR_STRICT_MODEL_VALIDATION). Logic is correct and the 11 tests cover precedence, fallback, whitespace-as-unset, warning emission, strict-mode raise, and case-insensitivity.

Non-blocking: _validate_model_config() calls _provider.get_context_length(...) with a # type: ignore[attr-defined], which suggests the metadata-provider Protocol doesn't declare that method — worth adding it to the Protocol for type safety. Also note _validate_model_config() runs at import time, so strict mode aborts process startup (intended, just calling it out).