feat(rotatui): charm stack upgrade — filter menus, glow peek, skate memory

[drawmeanelephant]

Summary

• Main menu swapped from gum choose to gum filter (fuzzy search over 19 rituals) with safe Esc handling under set -e (fixes latent hard-exit on abort)
• Template picker in the New Tomb wizard now uses gum filter and drops the stale hardcoded fallback list — falls back to globbing bones/templates/*.html so all 15+ themes appear
• New Peek At A Tomb ritual: gum file picker rooted at home/content/ + glow -p markdown preview (degrades gracefully if glow is absent)
• Skate memory: remembers last custom subdirectory (prefilled next run) and last scaffolded slug after successful scaffolds; silent no-op without skate
• Startup stack notice lists missing optional companions (glow/skate) with a brew install line; gum remains the only hard requirement
• spooky_spin intentionally untouched

Dependency model

• Required: gum
• Optional (soft-checked): glow (tomb peek), skate (scaffold memory)
• Not runtime: vhs (demo tapes only, follow-up)

Validation

• bash -n + shellcheck clean
• bash rotkeeper.sh test: 3/3 layouts + contract/DIP/regression suites passed (409s)
• bash rotkeeper.sh status healthy