feat: add per-model compatibility overrides

[AviBackToBlack]

Summary

Implement the second production slice of the v0.3 model-selection/override contract from #19.

This PR:

• adds strict exact-keyed model_overrides configuration
• validates a closed whitelist of capability booleans, token limits, supported OpenAI parameters, and reasoning effort sets
• rejects unknown fields, invalid values, selector-uncovered targets, and contradictory override tables
• applies overrides only after deterministic model-group aggregation, preserving exact-template identity and deployment evidence
• retains original aggregate evidence beside configured and effective override values in explain
• applies dependency closure after overrides (supports_reasoning=false disables reasoning transport/efforts; supports_function_calling=false prevents parallel calls)
• allows explicit context overrides to unblock otherwise-unsynthesizable foreign multi-deployment groups
• keeps exact Codex context fields authoritative even when max_input_tokens evidence is overridden
• keeps foreign Codex web search disabled even when effective web-search evidence is true
• repairs generated-field provenance so configured assertions are not misreported as LiteLLM observations
• preserves strict=false semantics when a selector-covered override target is temporarily absent from the current LiteLLM payload
• documents model_globs and model_overrides in README/config.example now that both v0.3 slices are implemented

Tests

Regression coverage includes config validation/normalization, selector-covered targets, contradiction handling, exact capability restoration, exact reasoning restriction including medium/high levels, reasoning/function dependency closure, foreign context blocker replacement, foreign web-search hard-disable, foreign verbosity transport, exact context ownership, CLI explain original/effective evidence, and strict=false absent-target behavior.

Refs #5

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