feat(cli): add --cwd flag to web and serve commands

[temujin9]

Summary

The opencode web and opencode serve commands did not preserve the working directory, causing process.cwd() to return an unexpected value (often /) when the server handles requests. This resulted in sessions being created with worktree: "/" instead of the intended project directory.

Changes

• Added --cwd flag to both web and serve commands
• Commands now call process.chdir() before starting the server, following the same pattern used by the TUI command (thread.ts)

Testing

• Typecheck passes
• Manual verification: --cwd option appears in --help output for both commands

Root Cause

The server middleware in server.ts (line 249) determines the directory per-request:

const directory = c.req.query("directory") || c.req.header("x-opencode-directory") || process.cwd()

Unlike the TUI command which calls process.chdir(cwd) before starting, the web and serve commands did not set the working directory, causing the fallback to process.cwd() to return an unintended value.