feat(core): per-agent subagent_depth override

[M4buAO]

Adds optional subagent_depth field to agent config (.md frontmatter or opencode.json). When set, overrides the global subagent_depth for that specific agent. Falls back to global -> 1 if unset.

core/config/agent.ts: schema field + KNOWN_KEYS
opencode/agent/agent.ts: runtime schema + state builder mapping
opencode/tool/task.ts: per-agent lookup before global fallback
opencode/test/tool/task.test.ts: 5 new test cases

Issue for this PR

Closes #

Type of change

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

What does this PR do?

The recent subagent_depth config (PR #37124) was global only — you set one limit for all agents. This PR makes it per-agent so you can give specific subagents a higher nesting limit while keeping everyone else at the default.
The change is small: added subagent_depth as an optional field in the agent config schema, plumbed it through the runtime state builder, and made the depth check in the task tool prefer the current agent's value over the global one. If an agent doesn't set it, it falls back to the global config, then to 1 — same behavior as before.

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How did you verify your code works?

Ran tests.

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it's not.

Checklist

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

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