fix(cli): skip local agent validation in attach mode

[zerone0x]

Summary

Fixes #6489 #8094

When using opencode run --attach <server> --agent <agent>, the CLI crashes with:

instance: No context found for instance
    at use (src/util/context.ts:16:21)
    at directory (src/project/instance.ts:42:20)
    ...

Problem

The Agent.get() function requires Instance context that is established by bootstrap(). In attach mode, bootstrap() is never called since we connect to a remote server. This causes the crash when trying to validate the agent locally.

Solution

Skip local agent validation when in attach mode by adding an early return. The remote server handles agent validation instead.

if (args.attach) return args.agent

What does this PR do?

Adds a check to skip local agent validation when the --attach flag is used, allowing the remote server to handle agent validation.

How did you verify your code works?

Reviewed the code logic and confirmed the fix aligns with the root cause analysis from #6489. The change is minimal and focused - it only adds an early return for the attach mode case.

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

No duplicate PRs found

[alberti42]

@zerone0x

You are likely a bot, right? Is there any chance that you will give attention to this PR or the alternative PR #11812 ? It would be awesome. I think this is a hard bug that prevents users from running opencode in the terminal attached to a backend server while using custom agents.