perf: parallelize picker annotations and add spinners for slow git ops

[RodrigoEspinosa]

Why

Two issues with the recent additions to wt:

• The interactive picker (and wt -l) felt slow on repos with many worktrees.
• Deleting a worktree appeared to freeze until it finished, with no feedback.

The picker's cost was structural, not the language: annotate_worktrees ran git status + git rev-list serially, once per worktree, before fzf even appeared.

What changed

Parallel + streaming picker

• Split per-worktree decoration into annotate_one, fanned out with xargs -P across all cores, streaming rows into fzf as they finish. N × git status serial → ~1× wall-clock, and fzf paints immediately.
• Branch/path are passed as NUL-delimited args (-0 -n2) so names/paths with spaces survive and the tab separator can't be mangled (this had also been silently breaking the ! dirty flag).
• Trade-off: row order is now non-deterministic; fzf sorts on match anyway, so it's invisible in the picker.

Spinners for slow git operations

• A spinner runs while git worktree remove, git worktree add, and wt pr (network fetch + checkout) execute.
• Non-TTY (scripts/CI/tests): falls back to a plain run with a one-line wt: <action>... breadcrumb. stdout stays reserved for the result path; the cursor is restored on Ctrl-C; exit codes propagate unchanged.

Version bumped to 0.6.1; CHANGELOG updated.

Note on stack

I looked into rewriting in Go/Rust for speed — it wouldn't help. The time is git stat-ing files, not Bash (measured startup: 0.002s). A port shelling out to git the same way would be just as slow. The only real reason to switch would be single-binary distribution, which isn't what these issues were about.

Verification

• All 57 bats tests pass; shellcheck clean.
• Smoke-tested live: parallel annotate with correct */!/↑↓ flags, spaced base-dir paths render as one field, create emits only the path on stdout, TTY spinner animates + cleans up.

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