feat: grid override file — .codedb/model_grid.toml for per-role overrides

[justrach]

Parent: #257

What

Per-role model overrides loaded from .codedb/model_grid.toml:

# Generated by evolve_agent after 10 runs on this codebase.
[orchestrator]
model = "sonnet"
rationale = "Haiku failed to decompose complex monorepo tasks correctly."

[mutator]
model = "opus"
rationale = "Opus produces 40% higher fitness organisms on this codebase."

Highest priority in the resolution chain — overrides base grid and config defaults.

Can be hand-written or generated by the evolutionary grid tuning (Phase 2, Codegraff Pro feature).

Depends on

• Model grid (#TBD)
• User config (#TBD)

[justrach]

Blocked by: #262.

[justrach]

Blocked by: #265.