Add the persistence layer for tracking archived git worktrees:

- `ArchivedGitWorktree` struct with `staged_commit_hash` and
`unstaged_commit_hash` fields to precisely identify WIP commits
- DB migrations for `archived_git_worktrees` and
`thread_archived_worktrees` (join table) tables
- CRUD operations: create, link to thread, query by thread, delete, mark
as restored
- `Column` impl for deserializing `ArchivedGitWorktree` from DB rows
- Tests for two-SHA round-trip, create/retrieve, delete, restore flag,
multi-thread linking, and multiple worktrees per thread

Part 2 of 3 in the persist-worktree stack. Stacked on #53213. This is
pure data model — nothing is wired up yet.

Release Notes:

- N/A

---------

Co-authored-by: Anthony Eid <anthony@zed.dev>