`opencode attach`: File path duplicates client's relative directory, causing ENOENT

[ZergRocks]

Description

When using opencode attach from a subdirectory of the server's workspace, file references using @ syntax cause the client's relative path to be duplicated in the final resolved path.

Steps to reproduce

1. Start the server from a parent directory:

   cd ~/Development
   opencode serve --port 9999

2. From another terminal, navigate to a subdirectory and attach:

   cd ~/Development/saturdays
   opencode attach http://localhost:9999

3. Reference a file using @ that exists in ~/Development/saturdays/docs/:

   @docs/report.md
   

4. Server throws ENOENT with a duplicated path:

   ENOENT: no such file or directory, stat '/Users/an/Development/saturdays/saturdays/docs/report.md'
   

Expected behavior

The file should be resolved to:

/Users/an/Development/saturdays/docs/report.md

Actual behavior

The path becomes:

/Users/an/Development/saturdays/saturdays/docs/report.md

The subdirectory saturdays appears twice - once from the client's cwd context and once from the path resolution.

Environment

• OpenCode version: 1.0.126
• OS: macOS
• Server location: ~/Development (via opencode serve --port 9999)
• Client attach location: ~/Development/saturdays (via opencode attach http://localhost:9999)

[rekram1-node]

good find we will fix

[oussamadouhou]

PR #6413 fixes --dir parameter handling by sending x-opencode-directory header to the server. This should prevent path duplication when attaching from subdirectories.