sp open: interactive fuzzy picker when no id is given (#10)

[rwrife]

Closes #10

What changed

sp open with no id now launches an interactive picker over the live scratches instead of requiring an id up front.

New internal/picker package (stdlib-only, no new deps)

• match.go — pure, I/O-free fuzzy matcher. Case-insensitive subsequence match over each scratch's id + name + tags, ranked with bonuses for contiguous runs and word-boundary hits (so tdo → todo, and a clean prefix outranks a scattered match). Fully unit-tested.
• select.go — front-ends chosen in priority order:
  1. fzf, when installed + on a TTY (the issue's "detect and defer to fzf if present").
  2. Built-in filter prompt on a TTY without fzf: numbered list, type to fuzzy-filter, number to pick, blank line takes the top match, q/Esc/Ctrl-C/EOF cancels.
  3. One-shot numbered prompt when not a TTY — the required graceful degradation for pipes/scripts.

Wiring

• sp open is now MaximumNArgs(1): sp open <id> keeps its exact old behavior; sp open (bare) runs the picker. Added --no-fzf to force the built-in picker.
• Extracted openScratch as the shared tail so a picked scratch opens identically to an explicit id (one $EDITOR launch site preserved).
• render.PickerLabel renders each row (id, name, age, expires-in, tags) — presentation stays in render; the picker owns interaction only. Plain (no ANSI) since the labels are also the fuzzy-match/fzf input.

Acceptance criteria

• sp open with no id opens an interactive selector (name, age, expires-in, tags visible)
• Keyboard-driven fuzzy filter; Enter/number opens the highlighted scratch in $EDITOR
• Works without an external fzf dependency (built-in prompt); detects and defers to fzf if present
• Esc/Ctrl-C/q cancels cleanly with no changes
• Degrades to a numbered prompt when not a TTY

Notes / deviations

• I went with a dependency-light built-in prompt rather than pulling in bubbletea/huh. Rationale: keeps go.mod unchanged (no go mod tidy churn, smaller supply-chain surface) and makes the whole selection flow headlessly unit-testable. The interaction still satisfies "keyboard-driven fuzzy filter" without raw-mode TUI. Happy to swap in a bubbletea TUI in a follow-up if you'd prefer the richer full-screen feel.
• The "extend to sp cat/rm/resurrect" idea from the issue notes is intentionally not in scope here; the picker package is structured so those can reuse Select later.

Testing

• 22 new tests across internal/picker (matcher + all three front-ends incl. fzf via injected fakes, and every cancel path) plus internal/cli (sp open picker end-to-end) and internal/render (label).
• go build ./... && go vet ./... && go test ./... all green; gofmt clean; go mod tidy produces no diff.