[feature] Fuzzy picker for sp open

[rwrife]

Pitch: Typing full ids is friction. sp open (with no argument) launches an interactive fuzzy picker over live scratches; pick one, it opens.

Acceptance criteria

• sp open with no id opens an interactive selector (name, age, expires-in, tags visible)
• Keyboard-driven filter/fuzzy match; Enter opens the highlighted scratch in $EDITOR
• Works without an external fzf dependency (use a Go TUI lib, e.g. bubbletea/huh), but detect and defer to fzf if present
• Esc/Ctrl-C cancels cleanly with no changes
• Degrades to a numbered prompt when not a TTY-capable terminal

Notes

• Same idea can later extend to sp cat/sp rm/sp resurrect.

[rwrife]

Opened #24 with the interactive fuzzy picker for sp open (bare, no id).

Highlights: new stdlib-only internal/picker (fuzzy subsequence matcher + three front-ends → fzf when present, built-in filter prompt on a TTY, numbered fallback when piped), --no-fzf flag, and Esc/Ctrl-C/q cancels cleanly. All 5 acceptance criteria covered; 22 new tests, build/vet/test/gofmt/tidy all green.

One deviation to flag: I used a dependency-light built-in prompt instead of bubbletea/huh (keeps go.mod untouched and the flow headlessly testable). Can swap in a full-screen TUI as a follow-up if preferred.