[feature] sp tui — interactive full-screen scratch browser

[rwrife]

Pitch

The 2026 "terminal renaissance" rewards small tools that offer an optional TUI (Posting, lazygit, serpl, etc.). scratchpatch is currently pure command + colorized table — great for scripting, but browsing/managing many scratches means eyeballing sp ls and copy-pasting IDs. sp tui adds an optional full-screen Bubble Tea browser over the same store: arrow-key through live + morgue scratches, preview content in a side pane, and act on the selection (open, rm→morgue, resurrect, promote, pin) without ever leaving the terminal.

This is explicitly a v0.2 capability — the v0.1 PLAN.md deliberately deferred a TUI ("no full-screen UI" in M-scope). Now that the command surface is stable, the TUI is a thin interactive shell over existing store/index/render logic, not a rewrite.

Why it fits

• Reuses internal/store, internal/index, internal/ttl, and the existing secret-scan + expiry classification. No new source-of-truth.
• Charm stack (lipgloss) is already a dependency; bubbletea + bubbles are the natural companions.
• Strictly additive: scripting/--json paths untouched; sp tui is opt-in and degrades to a clear error when stdout is not a TTY.

Acceptance criteria

• sp tui launches a full-screen browser listing live scratches (id, name, age, expires-in, size, 🔑 secret marker), color-coded by expiry like sp ls.
• Toggle to the morgue view (e.g. tab/m) showing purge countdowns.
• Right-hand preview pane shows the selected scratch's content (respects/notes secret scratches; does not dump raw credentials by default).
• Key-bound actions on the selection: open in $EDITOR (suspend/restore TUI), rm (→ morgue), resurrect, promote, with a confirmation for anything that moves files. All go through existing store operations (still two-step / never hard-delete from the TUI).
• Live filter/search box (by name/tag).
• Graceful, clear error when not attached to a TTY ("sp tui needs an interactive terminal; use sp ls/sp ls --json for scripting").
• Help/keybinding footer; q/esc quits cleanly and restores the terminal.
• No changes to any --json or plain command output; TUI code isolated (e.g. internal/tui).
• Tests where practical (model update/transition unit tests via Bubble Tea's test helpers); go build/go vet/go test ./.../gofmt clean.
• README documents sp tui and its keybindings.

[rwrife]

Implemented in #31 — sp tui full-screen browser (live/morgue panes, expiry-tinted list, secret-safe preview, filter, open/rm/resurrect with y/N confirms, TTY guard). Isolated in internal/tui; no scripting/--json changes. Phase 1 next cycle will handle the merge.