Add a config-level default agent model

[FreshlyBrewedCode]

What to build

The agent model has a four-level precedence chain with no config rung. Today it resolves as
per-call option → schedule override → workflow default → a DEFAULT_MODEL constant hardcoded in
runtime/run.ts. A project cannot set its own default model anywhere.

That constant is also already stale: it reads opencode-go/deepseek-v4.1-flash, while AGENTS.md
says the preferred model for real agent tests is opencode-go/big-pickle. A provider-qualified
model id living in the runtime is the wrong home for a value that is plainly project policy —
factory.config.ts already owns repo, identity, concurrency, retention and schedules.

Add an agent.model default to FactoryConfig, so the chain becomes run request → schedule →
workflow → config → built-in fallback. This mirrors the precedence rule epic #19 already
established for everything a schedule can override.

Keep a built-in fallback for the config-less paths: factory run without a project config, and a
daemon started with no config, both still have to pick something.

Acceptance criteria

• factory.config.ts accepts an agent.model default, validated at load like the rest of the config
• Precedence resolves as run request > schedule > workflow > config > built-in fallback, with a test covering each rung
• The runtime constant is no longer the only place a default model is expressed
• The built-in fallback is current rather than stale, and agrees with what AGENTS.md recommends
• Runs started without any config still work
• bun run check passes

Blocked by

None (can start immediately).