feat(core): expose workspace adaptor and VCS branch hooks to plugin system

[lytedev]

Issue for this PR

Loosely related to #5008. The installAdaptor() function and Adaptor interface already exist internally with a comment saying "we will likely allow this in the future" — this PR implements that by exposing it to the plugin system.

Type of change

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

What does this PR do?

Exposes workspace adaptor registration and VCS branch detection to the plugin system, so plugins can register alternative workspace backends (e.g. jj workspaces) alongside the built-in git worktree adaptor.

I use jj (jujutsu) for version control. jj has its own workspace concept (jj workspace add/forget) that's incompatible with git worktrees — jj git init --colocate refuses to run inside a git worktree, and running raw git worktree commands behind jj's back corrupts state.

Changes (all additive, no existing behavior modified):

1. workspace.adaptor hook (record-type, same pattern as tool): plugins register adaptor implementations keyed by type name. After plugins load, each is registered via the existing installAdaptor().

2. detect() method on adaptors: plugin adaptors can provide detect(directory) => boolean. The /experimental/worktree create endpoint now calls detectAdaptor() first — if a plugin claims the directory, it creates a workspace through that adaptor instead of the default git worktree path.

3. vcs.branch hook (pipeline): lets plugins provide branch info for non-git VCS. Also used as fallback when git rev-parse fails.

4. WorkspaceAdaptor and WorkspaceAdaptorInfo types exported from @opencode-ai/plugin.

5. workspaceFetch helper on PluginInput: proxies requests through the built-in WorkspaceServer so plugin adaptors don't need internal imports.

6. GET /experimental/workspace/types endpoint: returns available adaptor types and auto-detected adaptor for the current project.

7. TUI workspace creation dialog: dynamically fetches available types from the API instead of hardcoding "worktree".

8. installAdaptor() cleanup: accepts both internal Adaptor and plugin WorkspaceAdaptor types cleanly, removed @ts-expect-error. getAdaptor() throws a descriptive error for unknown types instead of crashing.

How did you verify your code works?

• bun typecheck passes clean for both packages/opencode and packages/plugin
• All changes are additive with no altered code paths for existing behavior
• Built and deployed a jj workspace plugin that exercises both hooks — workspace creation via jj workspace add and branch detection via jj log

Screenshots / recordings

No UI changes beyond the workspace creation dialog showing dynamically fetched types instead of a hardcoded list.

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. 👍

[lytedev]

Mostly superseded by #15895 it looks like! Will see if there are any gaps for my jj workspace plugin...

[github-actions]

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