Custom agent with "name" field breaks 'opencode run --agent <key>' resolution (agent=undefined -> Unexpected server error)

Bug: custom agent with a name field breaks opencode run --agent <key> resolution

Symptom

Defining a custom agent in opencode.json with a name field (display name different from the config key) makes opencode run --agent <key> fail with:

Error: {
  "name": "UnknownError",
  "data": { "message": "Unexpected server error. Check server logs for details.", "ref": "err_..." }
}

The created session is logged with agent=undefined. Passing the display name instead (--agent "<display name>") gives "agent not found. Falling back to default agent".

Repro

// opencode.json (with or without plugins — reproduces with `--pure`)
{
  "agent": {
    "tmp-ocb": {
      "mode": "primary",
      "name": "Tmp OCB Display",
      "description": "test"
    }
  }
}

opencode run --agent tmp-ocb "Reply OK"        # → Unexpected server error (agent=undefined)
opencode run --agent "Tmp OCB Display" "Reply OK"  # → "agent not found. Falling back to default agent"

Removing the name field fixes it: --agent tmp-ocb then resolves and runs normally.

Isolation

Reproduces with --pure (no external plugins loaded), so this is an opencode core issue, not plugin-related.

Notes

• name is not documented in the AgentConfig schema (https://opencode.ai/config.json) yet packages/opencode/src/agent/agent.ts reads it (item.name = value.name ?? item.name). The run-path agent lookup does not handle a key whose display name differs from the key.
• Environment: opencode 1.18.18 (standalone binary), Linux.

Expected

Either opencode run --agent <key> resolves the agent regardless of a differing name field, or the name field is documented/validated explicitly (rejected if unsupported) instead of silently breaking the lookup.