fix: resolve git worktrees to main repo in workspace filter (#3366)

Fixes #3364

## Summary

When using `filter_mode = "workspace"`, each git worktree gets its own
isolated history scope instead of sharing history with the main
checkout.

## Root cause

`in_git_repo()` walks up the directory tree looking for a `.git` entry.
In a worktree, `.git` is a file (not a directory) containing a `gitdir:`
pointer back to the main repo's `.git/worktrees/<name>`. Since
`has_git_dir()` just checks `.exists()`, the worktree's own path becomes
the workspace root, and the `WHERE cwd LIKE '<root>%'` filter isolates
its history from the main repo and other worktrees.

## Fix

Add `resolve_git_worktree()`, which reads the `.git` file when it's not
a directory, parses the `gitdir:` pointer, and walks back up to find the
parent containing a real `.git` directory. No new dependencies -- just a
bit of file reading and path traversal.

## Testing

Two new tests in `utils::tests`:
- `in_git_repo_regular` -- baseline that normal repos still resolve
correctly
- `in_git_repo_worktree_resolves_to_main_repo` -- creates a simulated
worktree layout and verifies it resolves to the main repo root

Both pass locally, along with the full `atuin-common` test suite and
`cargo clippy -- -D warnings`.

---

Disclosure: I'm a Rust novice, and I put this together with help from
Claude Code. I'm eager to learn more, so please let me know if anything
doesn't feel like idiomatic Rust!

## Checks
- [x] I am happy for maintainers to push small adjustments to this PR,
to speed up the review cycle
- [x] I have checked that there are no existing pull requests for the
same thing

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