sp tui — interactive full-screen scratch browser (#26)

[rwrife]

Closes #26

Adds sp tui: an optional, full-screen Bubble Tea browser over the same store the CLI already uses. Strictly additive — no --json/scripting path changes, no new source of truth. The model reads scratches through a small Backend interface (satisfied by *store.Store) and drives the existing MoveToMorgue/Resurrect/open-in-$EDITOR operations, so it can never hard-delete or diverge from sp ls.

What changed

• internal/tui/ — new isolated package: Model (Bubble Tea) with live/morgue panes, expiry-tinted list, side preview pane, live filter, and two-step y/N confirmation for any file move. Content/logic seam behind Backend + injected Now/OpenEditor for deterministic tests.
• internal/cli/tui.go — sp tui command; refuses to launch when stdout/stdin isn't a TTY (clear pointer to sp ls); reuses the exact $EDITOR launcher sp open uses; alt-screen program.
• Wired into root; go get bubbletea v1.3.4 + bubbles v0.20.0.
• README documents sp tui and its keybindings.

Acceptance criteria

• Full-screen browser of live scratches (id, name, age, expires-in, size context, 🔑 secret marker), color-coded by expiry like sp ls.
• Toggle to the morgue view (tab/m) with purge countdowns.
• Preview pane for the selection; secret scratches show a redaction notice, never raw credentials.
• Key-bound open ($EDITOR suspend/restore), rm→morgue, resurrect, each through existing store ops with a confirmation for file moves; never hard-deletes from the TUI.
• Live filter by name/id/tag.
• Graceful, clear error when not attached to a TTY.
• Help/keybinding footer; q/esc quits and restores the terminal (alt-screen).
• No --json/plain-command output changes; TUI isolated in internal/tui.
• Model update/transition unit tests (fake backend, no real TTY); go build/vet/test ./.../gofmt clean.
• README updated.

Notes: promote-from-TUI was scoped out to keep the interactive surface to the reversible ops (open/rm/resurrect); promote stays a deliberate CLI action. Preview is height-bounded so huge scratches don't blow the frame.