fix(agent): skip parseModel when model is "inherit", trim whitespace

[yjlc-pc]

Issue for this PR

Closes #17890, #31141, #5623, #8896, #23908

Type of change

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

What does this PR do?

Custom .md subagents with model: inherit (the default when no model is specified in frontmatter) crash with ProviderModelNotFoundError. The string "inherit" is passed directly to Provider.parseModel(), which splits on / and produces {providerID: "inherit", modelID: ""}. Since no provider named "inherit" exists, Provider.getModel() throws ProviderModelNotFoundError on every subagent invocation.

Additionally, some YAML frontmatter parsers (e.g. gray-matter) may leave trailing
on model strings, which also causes Provider.parseModel() to fail on otherwise valid model IDs.

Fix:

1. packages/opencode/src/agent/agent.ts: Skip Provider.parseModel when value.model === "inherit", leaving item.model undefined. The task.ts fallback (next.model ?? { modelID: msg.info.modelID, providerID: msg.info.providerID }) then correctly inherits the parent session model. Also .trim() the model string for trailing whitespace.
2. packages/opencode/src/provider/provider.ts: Add .trim() in parseModel for defense-in-depth against trailing whitespace.

How did you verify your code works?

• The agent.ts fix matches the approach in PR fix(agent): honor model:inherit in subagent frontmatter #17888 (same change, confirmed working by the original author)
• The provider.ts .trim() is a standard defensive programming practice for string inputs
• Both changes are minimal (2 lines total) and the logic is straightforward: skip parseModel when the value is the literal string "inherit", and strip surrounding whitespace

Screenshots / recordings

N/A - backend logic fix.

Checklist

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

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[yjlc-pc]

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