feat: add --dir flag to opencode run --attach

[athal7]

Summary

• Add --dir flag to opencode run command
• When used with --attach, passes directory to SDK client via x-opencode-directory header
• Enables sessions attached to a global server to run in the correct project directory

Use Case

When using opencode-pilot to spawn sessions that attach to a running OpenCode Desktop with worktree="/", sessions now run in the correct project directory instead of the server's home directory.

# Before: sessions run in wrong directory
opencode run --attach http://localhost:4096 "fix bug"
# Working directory: /Users/me (wrong!)

# After: sessions run in correct directory
opencode run --attach http://localhost:4096 --dir /path/to/project "fix bug"
# Working directory: /path/to/project (correct!)

Closes #7376