fix: improve implicit queue path resolution in worktrees

[DerekStride]

Summary

This changes implicit queue path resolution to follow git worktree semantics instead of treating the default queue path as purely cwd-relative.

Resolution order is now:

1. respect --queue exactly
2. respect SQ_QUEUE_PATH exactly
3. otherwise, if inside a git worktree, search upward from the current directory to the current worktree root for the nearest existing .sift/issues.jsonl
4. if none exists in the current worktree and this is a linked worktree (git-dir != git-common-dir), mirror that same upward search in the main worktree
5. if still none exists, fall back to <cwd>/.sift/issues.jsonl
6. outside git, fall back to .sift/issues.jsonl under the current directory

Why

The old default of .sift/issues.jsonl worked like a cwd-relative path. That caused two problems:

• running sq from a nested directory could accidentally create a new queue instead of finding an existing project or subproject queue
• in linked git worktrees, the intended queue may exist in the main worktree while being absent from the linked checkout

Implementation notes

• queue resolution logic was separated from git command execution so the behavior is testable as pure path-selection logic
• git shelling remains only in the thin adapter that gathers:
  • --show-toplevel
  • --git-dir
  • --git-common-dir
• the lookup behavior itself is now covered by unit tests instead of integration tests that shell out to real git

Examples

Simple linked-worktree example

Consider:

• main worktree root: /projects/sq
• linked worktree root: /projects/sq.feature
• cwd: /projects/sq.feature
• existing queue: /projects/sq/.sift/issues.jsonl

sq first checks the linked worktree itself:

• /projects/sq.feature/.sift/issues.jsonl

If that file does not exist, it mirrors the cwd into the main worktree and checks:

• /projects/sq/.sift/issues.jsonl

Since that queue exists, sq uses /projects/sq/.sift/issues.jsonl.

Example: Nested Directories

For linked worktrees, git rev-parse --git-dir and git rev-parse --git-common-dir distinguish whether the current checkout is the main worktree or a linked one.

Consider:

• current worktree root: /trees/feature
• main worktree root: /repo
• cwd: /trees/feature/src/packages/alpha/client

First, sq searches upward in the current worktree for the nearest existing queue:

• /trees/feature/src/packages/alpha/client/.sift/issues.jsonl
• /trees/feature/src/packages/alpha/.sift/issues.jsonl
• /trees/feature/src/packages/.sift/issues.jsonl
• /trees/feature/src/.sift/issues.jsonl
• /trees/feature/.sift/issues.jsonl

That covers the nested-directory case: if a narrower queue already exists in the current worktree, it will be found and used.

If none of those exist and this is a linked worktree, sq mirrors the relative path src/packages/alpha/client into the main worktree and performs the same upward search there:

• /repo/src/packages/alpha/client/.sift/issues.jsonl
• /repo/src/packages/alpha/.sift/issues.jsonl
• /repo/src/packages/.sift/issues.jsonl
• /repo/src/.sift/issues.jsonl
• /repo/.sift/issues.jsonl

If /repo/src/packages/.sift/issues.jsonl exists, that file is used.

If nothing exists in either place, sq falls back to the current working directory:

• /trees/feature/src/packages/alpha/client/.sift/issues.jsonl