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

[filipeandre]

Summary

• PlanExitTool now resolves the model for the build agent using Agent.get("build") config, falling back to the last session model if none is configured
• PlanEnterTool does the same for the plan agent
• Renames the internal getLastModel helper to lastModel (style) and extracts a resolveModel helper
• Adds tests covering both the configured-model path and the fallback path

Motivation

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 that config and always using the last model seen in the session. This PR wires up Agent.get() so the configured model is respected.

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The following comment was made by an LLM, it may be inaccurate:

Potential Duplicate Found:

• PR fix(plan): resolve model from agent config in plan tools #18163: "fix(plan): resolve model from agent config in plan tools"
  • fix(plan): resolve model from agent config in plan tools #18163
  • This appears to be the same PR (note: current PR is fix(plan): resolve model from agent config in plan tools #18161, but fix(plan): resolve model from agent config in plan tools #18163 also has the identical title and appears in search results)

This suggests PR #18163 may be a duplicate or related PR addressing the exact same issue. You should verify whether this is an alternate version of the same fix or if both PRs should exist.