feat: add GOOSE_SUBAGENT_MODEL and GOOSE_SUBAGENT_PROVIDER config options

[tlongwell-block]

Adds GOOSE_SUBAGENT_MODEL and GOOSE_SUBAGENT_PROVIDER to the subagent provider resolution chain in resolve_provider(). Uses Config::global().get_param() (env var with config.yaml fallback), matching the GOOSE_LEAD_*/GOOSE_PLANNER_* pattern.

Priority: delegate params > recipe/agent settings > these config options > parent session inheritance.

One or both can be set. When neither is set, behavior is unchanged.

[tlongwell-block]

/goose

[github-actions]

Summary: This PR adds GOOSE_SUBAGENT_MODEL and GOOSE_SUBAGENT_PROVIDER config options to the subagent provider resolution chain. The implementation is clean, minimal, and correctly follows the established GOOSE_LEAD_*/GOOSE_PLANNER_* pattern using Config::global().get_param().

✅ Highlights

1. Consistent pattern usage (summon.rs:1410-1413, 1438-1439): Uses Config::global().get_param() which provides env var lookup with config.yaml fallback, matching how GOOSE_LEAD_* and GOOSE_PLANNER_* are handled elsewhere in the codebase.

2. Correct priority chain: The placement in the resolution chain follows the documented priority (delegate params > recipe/agent settings > config options > parent session inheritance).

3. Minimal, focused change: Only 8 lines added, exactly where needed. No unnecessary refactoring or scope creep.

🟢 Suggestions

1. Pre-existing inconsistency (summon.rs:1454): The nearby resolve_max_turns() function uses std::env::var("GOOSE_SUBAGENT_MAX_TURNS") directly instead of Config::global().get_param(). This means GOOSE_SUBAGENT_MAX_TURNS only reads from env vars, not config.yaml, while the new GOOSE_SUBAGENT_PROVIDER/GOOSE_SUBAGENT_MODEL options support both. Consider aligning this in a follow-up PR for consistency:

   // Current (env var only):
   std::env::var("GOOSE_SUBAGENT_MAX_TURNS")
   
   // Could be (env var + config.yaml):
   Config::global().get_param::<usize>("GOOSE_SUBAGENT_MAX_TURNS")

   This is not blocking as it's outside the scope of this PR.

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Review generated by goose