test(task): lock in markdown subagent descriptions

[SpencerJung]

Summary

• add a regression test for markdown-defined subagents loaded from .opencode/agents/*.md
• verify the frontmatter description survives into Agent.list()
• verify the Task tool description renders that description instead of any generic fallback text

Why

Issue #27831 reports that markdown-defined subagents can fall back to a generic manual-only description in the Task tool even when description is present in frontmatter. On current dev, the production code path already preserves that field, but there was no regression coverage for the markdown-defined agent path.

Closes #27831

Verification

• lsp_diagnostics on packages/opencode/test/tool/task.test.ts -> no diagnostics
• �un run test test/tool/task.test.ts (from packages/opencode) -> 16 pass, 0 fail
• attempted �un install in the worktree, but it failed on ree-sitter-powershell because this Windows environment does not have the required Visual Studio C++ toolchain; no lockfiles were modified

[github-actions]

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[github-actions]

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

Potential Related PR Found:

• PR test(agent): lock in markdown frontmatter description propagation #28086: test(agent): lock in markdown frontmatter description propagation
  • test(agent): lock in markdown frontmatter description propagation #28086

Why it's related: This PR appears to cover similar territory—testing markdown frontmatter description propagation for agents. PR #29377 focuses on the Task tool description rendering, while #28086 appears to test the Agent.list() path. They may be addressing complementary aspects of the same issue (#27831) or one could be a predecessor/related fix.

I recommend checking if PR #28086 is already merged and whether it addresses the regression test needs or if they cover different test scenarios.

[github-actions]

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