feat(cli): add ACP attach support

[shekohex]

Issue for this PR

Closes #12853

Type of change

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

What does this PR do?

Adds --attach to opencode acp so ACP can bridge stdio to an existing opencode serve backend instead of always starting a local server.

It keeps the current local startup path unchanged, forwards --cwd in both modes, and updates the ACP/CLI docs plus the localized ACP/CLI pages.

How did you verify your code works?

• bun typecheck
• printf '%s\\n' '{\"jsonrpc\":\"2.0\",\"id\":1,\"method\":\"initialize\",\"params\":{\"protocolVersion\":1}}' | bun run ./src/index.ts acp
• printf '%s\\n' '{\"jsonrpc\":\"2.0\",\"id\":1,\"method\":\"initialize\",\"params\":{\"protocolVersion\":1}}' | bun run ./src/index.ts acp --attach http://127.0.0.1:4105 with bun run ./src/index.ts serve --port 4105

Screenshots / recordings

N/A

Checklist

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

[github-actions]

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

Based on my search, I found a potentially related PR:

#12743 - "feat(opencode): add backend server selection options for acp mode"

This PR appears to address similar functionality around allowing ACP to connect to backend servers instead of always starting locally, which is closely related to the --attach feature being added in PR #18272.

However, PR #12743 appears to be from an earlier iteration or different approach. You may want to review it to ensure there's no overlapping implementation or if it was previously closed/superseded.