[FEATURE]: Add /cd slash command for runtime working directory switching

[yangelaboy]

Feature hasn't been suggested before.

• I have verified this feature I'm about to request hasn't been suggested before.

Describe the enhancement you want to request

Related: #2177, #15220

Add a /cd slash command that allows changing the working directory at runtime without restarting OpenCode. This is a common workflow need when navigating between projects or worktrees mid-conversation.

Proposed behavior

• /cd <path> — switch to the given directory (supports relative paths, absolute paths, ~ expansion)
• /cd without arguments — return to the initial startup directory
• Appears in slash command autocomplete list
• When switching outside the current project root, show a confirmation dialog
• Record the directory change in the session as a shell command (cd <path> display + pwd output)

Why this is useful

Users frequently need to navigate between related projects, monorepo packages, or git worktrees during a single session. Currently the only option is to quit and restart OpenCode in the new directory. A lightweight /cd command avoids losing conversation context.

Implementation approach

• TUI-side only: resolve path, validate directory exists, update SDK's target directory, record in session
• Minimal changes (~150 lines added across 7 files), no server-side changes needed
• A previous attempt (feat: add /cd command for changing working directory #7266, 3400+ lines) was closed — this approach is significantly smaller and focused

Environment

• OS: macOS (darwin-arm64)
• Terminal: Ghostty

[github-actions]

This issue might be a duplicate of existing issues. Please check:

• [FEATURE]: /cd command: add to slash command autocomplete list + interactive directory picker #15220: [FEATURE]: /cd command: add to slash command autocomplete list + interactive directory picker — nearly identical feature request for a /cd slash command
• Allow explicitly changing working directory #2177: Allow explicitly changing working directory — tracks the same underlying need for runtime working directory switching

(You've already referenced these in your issue, which is great! It may be worth consolidating discussion into one of those existing threads.)