fix(plan): resolve model from agent config in plan tools

[filipeandre]

Issue for this PR

Closes #9296

Type of change

• Bug fix
• New feature
• Refactor / code improvement
• Documentation

What does this PR do?

When a user configures a specific model for the build or plan agent (e.g. agent.build.model = "openai/gpt-4o"), the plan tools were ignoring it. Both PlanExitTool and PlanEnterTool were always falling back to the last model seen in the session history via getLastModel, so the build agent would inherit the plan agent's model instead of its own configured one.

This PR adds a resolveModel helper that calls Agent.get(agentName) first and uses the configured model if present, only falling back to the last session model when none is set. PlanExitTool resolves for the build agent and PlanEnterTool for the plan agent.

How did you verify your code works?

Added two tests in packages/opencode/test/tool/plan.test.ts:

• One verifies that when agent.build.model is configured, the message written by PlanExitTool uses that model.
• One verifies that when no model is configured, it falls back to the last session model.

Screenshots / recordings

N/A — not a UI change.

Checklist

• I have tested my changes locally
• I have not included unrelated changes in this PR

[github-actions]

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Please:

1. Open an issue describing the bug/feature (if one doesn't exist)
2. Add Fixes #<number> or Closes #<number> to this PR description

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[github-actions]

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