fix(attach): allow remote --dir

[gricha]

What does this PR do?

I spawn OpenCode directly on my dev servers in $HOME. I then try to attach to them from any of my machine, and because projects may be scattered and not at $HOME, I want to be able to use --dir parameter. In these examples docs is my hostname in Tailnet.

If I ran opencode attach docs:4096 --dir ./perry from my machine earlier, it would consider it a local path and try to push that in. Absolute paths would also be caught with issues like:

ENOENT: no such file or directory, chdir '/home/gricha/personal/opencode-gricha' -> '/home/workspace/perry'

I didn't want to break local-validation functionality you have, so I added gating for local vs. remote attach, and let remote pass through the dir to the upstream server.

How did you verify your code works?

opencode attach docs:4096 --dir ./perry
opencode attach docs:4096 --dir /home/workspace/perry
both work properly now - I also tested for whether they are able to load up skills/subagents from those directories after that change and it works fine.

Fixes #8981

[github-actions]

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[github-actions]

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

Potential Duplicate Found:

• PR Be able to attach to a remote directory #6250: "Be able to attach to a remote directory" - Be able to attach to a remote directory #6250

  This PR appears to address the same feature request: enabling the --dir parameter when attaching to a remote OpenCode instance. It's likely either a previous attempt, a related discussion, or an already-closed duplicate of the current PR fix(attach): allow remote --dir #8969.

[gricha]

I'd not consider this a duplicate, I think this is a much simpler fix of existing functionality that just makes sure that intended behavior works properly.

[gricha]

@rekram1-node @thdxr could I get a quick look at this? i spin up one server remotely on my docker containers right now, and having this validation disabled would let me start them in the right repositories, with proper skills/agents loaded up. I have to do a bit of a dance right now without it. sorry to bug multiple times about that