feat(tui): add session migration dialog

[sim590]

Issue for this PR

Closes #23249

Type of change

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

What does this PR do?

Adds a session migration dialog accessible via ctrl+o from the session list. This allows users to:

1. Recover orphaned sessions — When a project directory is renamed or moved (Sessions become orphaned when project directory is renamed #23248, Session lookup fails with NotFoundError when PTY spawned from non-git directory context #8538), sessions become invisible because their directory field points to a path that no longer exists. The migration dialog shows all sessions across all projects, marks orphaned ones with a yellow ! indicator, and allows moving them to the correct project.

2. Intentionally migrate sessions between projects — When work was started in the wrong project, users can move the session (with its full conversation history) to another project rather than starting over.

3. Delete orphaned sessions — Since orphaned sessions don't appear in the normal /sessions list, the migration dialog is the only place where they can be found and deleted (ctrl+d with double-press confirmation).

How it works:

• The dialog fetches all sessions via /experimental/session (bypassing the SDK's automatic directory filtering) and identifies orphans via a new GET /session/orphans endpoint.
• Orphan detection uses two criteria: (1) the session directory no longer exists on disk, (2) a global session whose directory is inside a known project worktree.
• Selecting a session opens a destination picker offering the current project (using the actual cwd), home directory, and all known projects.
• Migration updates project_id and directory on the session and its children (sub-agents) via a new POST /session/:sessionID/migrate route.
• After migration, the session list is refreshed so the migrated session appears immediately.

Files changed:

• dialog-session-migrate.tsx — New migration dialog component
• dialog-session-rescue.tsx — New destination picker component
• dialog-session-list.tsx — Added ctrl+o keybind to open migration dialog
• keybinds.ts — Added session_migrate keybind (default: ctrl+o)
• session/session.ts — Added listOrphans() and migrate() functions
• server/routes/instance/session.ts — Added orphans and migrate routes
• sdk/js/src/v2/gen/ — Regenerated SDK with new endpoints

Note: This does not fix the root cause of orphaned sessions (#23248). An automatic fix in fromDirectory() to update session directories when a project worktree changes would still be valuable, but involves subtleties (e.g. multiple clones of the same repository sharing a project ID).

Possible future improvements:

• Show a warning in the TUI when opening a migrated session, so the user can give context to the LLM about the project change.
• Inject a system message after migration to orient the LLM toward the new project context.

How did you verify your code works?

Tested manually on Linux (Alacritty terminal):

• Created orphaned sessions by renaming project directories (both cases from Sessions become orphaned when project directory is renamed #23248)
• Verified orphan detection (yellow ! indicator appears and persists after filtering)
• Migrated sessions between projects and verified they appear in /sessions immediately
• Deleted orphaned sessions and verified full cascade removal (session, messages, parts)
• Verified bun typecheck passes with zero errors

Screenshots / recordings

Migration dialog showing all sessions with orphaned ones marked with !:

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Destination picker after selecting a session:

Checklist

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

[sim590]

Update: Improved the orphan indicator UX:

• The ! orphan label in the footer was misleading — it looked like a keybind, as if pressing ! would "orphanize" a session
• Replaced it with a contextual note at the bottom: NOTE: ! means the session is orphan
• The note is only shown when orphaned sessions are actually present in the list
• The ! in the note is displayed in the same warning color as the gutter indicator for consistency