Support user-defined custom editors in the Open In picker

[kevinher7]

What Changed

Adds a customEditors array to server settings (settings.json) so any command can be offered in the "Open in" picker — e.g. a terminal editor wrapped in a terminal emulator:

"customEditors": [
  {
    "id": "nvim-ghostty",
    "name": "Neovim (Ghostty)",
    "command": ["ghostty", "-e", "nvim", "{path}"]
  }
]

• contracts: EditorId widens to built-in literals | custom:<slug> (Schema.TemplateLiteral, mirroring the existing script.<id>.run pattern); new CustomEditorDefinition with a {path} placeholder (target path is appended when no argument contains it); ServerSettings.customEditors defaults to [] so existing settings files and persisted localStorage editor ids decode unchanged.
• server: resolveEditorLaunch resolves custom: ids against definitions the shell.openInEditor RPC handler reads from server settings. Commands come exclusively from the server-side settings file — the client still only sends an editor id, so the security posture is unchanged.
• web: the picker lists custom editors (always shown — the user opted in explicitly; a missing command surfaces as an error toast instead of a silently hidden entry), preferred-editor resolution covers them everywhere (picker, file links, diagnostics "open logs", keybindings toast), and the existing mod+o / editor.openFavorite shortcut works once a custom editor is the last-used one.
• settings.json edits hot-reload through the existing settings watcher and stream to connected clients via the server config subscription — no restart needed.

Also includes a separate commit fixing a pre-existing formatting issue in Migrations.ts that made vp check fail on the base branch.

Why

There is currently no way to open a session's worktree in a terminal editor like Neovim — the editor list is a hardcoded set of GUI editors. Rather than hardcoding one more entry (ghostty+nvim), this makes the list user-extensible through the already-existing, file-backed, hot-reloading server settings pipeline, which keeps the change small: no new service, no new RPC, no client-supplied commands.

UI Changes

Custom editors appear as additional entries in the existing Open In dropdown (terminal icon + configured name). No layout changes; no screenshots included.

Testing

• 4 new resolveEditorLaunch unit tests: {path} substitution, append-when-no-placeholder, multi-argument substitution, unknown custom id fails
• vp run typecheck passes for all 15 packages; vp check passes
• Full server suite (1180 tests), shared (175), and web localApi (17) pass
• Local config prepared with ["/Applications/Ghostty.app/Contents/MacOS/ghostty", "-e", "nvim", "{path}"] in ~/.t3/dev/settings.json (not yet exercised through the running app)

Checklist

• This PR is small and focused
• I explained what changed and why
• I included before/after screenshots for any UI changes
• I included a video for animation/interaction changes

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