feat: add branch selector and VCS checkout APIs

[itskritix]

Issue for this PR

Closes #14265

Type of change

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

What does this PR do?

• Adds VCS API endpoints for branch listing and checkout (/vcs/branches, /vcs/checkout) with typed errors and SDK generation.
• Adds a branch/workspace selector in the session header to browse local and remote branches and switch from the UI.
• Prevents switching to branches already checked out in another worktree.
• Makes git worktree list --porcelain parsing CRLF-safe to avoid platform parsing issues.

How did you verify your code works?

• bun run typecheck in packages/opencode
• bun run typecheck in packages/app
• OPENCODE_SERVER_PASSWORD= bun run test in packages/opencode
• bun run test:unit in packages/app
• bun run build in packages/opencode
• bun run build in packages/app

Screenshots / recordings

• UI change: session header now includes a branch/workspace popover with grouped local/remote branches and an "in use" badge.
• I could not capture a screenshot from this CLI-only environment.

Checklist

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

[github-actions]

Thanks for updating your PR! It now meets our contributing guidelines. 👍

[Copilot]

Pull request overview

This PR adds branch switching capabilities to the OpenCode IDE by implementing VCS API endpoints and a UI branch selector component. The feature is integrated into the session header and includes worktree-aware checkout logic to prevent conflicts when branches are used in multiple worktrees.

Changes:

• Added vcs.branches and vcs.checkout API endpoints with typed error definitions (VcsBranchListFailedError, VcsBranchNotFoundError, VcsBranchCheckoutFailedError)
• Generated TypeScript SDK bindings for the new VCS endpoints
• Implemented a branch selector popover in the session header that displays local and remote branches, marks branches in use by other worktrees, and handles branch checkout

Reviewed changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
packages/opencode/src/project/vcs.ts	Added branches() and checkout() functions with worktree detection and git command execution
packages/opencode/src/server/server.ts	Registered /vcs/branches (GET) and /vcs/checkout (POST) routes with error mapping
packages/sdk/js/src/v2/gen/types.gen.ts	Generated TypeScript types for VcsBranch, VcsCheckoutInput, and response/error types
packages/sdk/js/src/v2/gen/sdk.gen.ts	Generated SDK methods vcs.branches() and vcs.checkout()
packages/app/src/i18n/en.ts	Added internationalization strings for branch groups, in-use badge, and error messages
packages/app/src/components/session/session-header.tsx	Implemented branch selector popover with search, grouping, and worktree conflict prevention

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[itskritix]

Addressed Copilot review notes in 5814ae23f:

• added server-side worktree occupancy guard in Vcs.checkout, so API calls now fail early with a clear VcsBranchCheckoutFailedError when the branch is checked out elsewhere
• hardened worktree porcelain parsing to also parse HEAD / detached lines while still mapping occupancy by branch refs
• added explicit-remote safety check: when selecting a remote branch with an existing local branch, we now verify upstream/commit alignment before switching to avoid silently selecting a divergent local branch

Also re-ran local checks (typecheck for app/opencode, app unit tests).