Add selectable character pack system

[keith-oak]

Summary

• Introduces a CharacterPack model following the existing PopoverTheme pattern — struct with static presets, allPacks array, static var current, menu switching by index tag
• Bruce & Jazz remain the default pack with zero behaviour change for existing users
• Adds a Droids pack (R2-Do2, C-3POa, BB-Gr8) with pixel art HEVC-with-alpha walking animations as an opt-in alternative
• New "Characters" submenu in the menu bar for runtime pack switching
• Selection persisted via UserDefaults (selectedCharacterPack key)
• Character toggle menu items are now dynamic — adapts to current pack's character count and names
• Each pack defines: characters (video, timing, colours, offsets), thinking/completion phrases, and onboarding text

Changes

File	What
CharacterPack.swift	New — CharacterConfig + CharacterPack structs with two presets
LilAgentsController.swift	Refactored to loadCharacters(from:) factory + switchPack(_:)
WalkerCharacter.swift	Added applyConfig(_:), phrases now read from current pack
LilAgentsApp.swift	Dynamic character items, "Characters" submenu, single toggleCharacter method
project.pbxproj	Added droid .mov files and CharacterPack.swift to build
walk-r2d2-01.mov	Pixel art R2-D2 waddle animation (369 KB)
walk-c3po-01.mov	Pixel art C-3PO stiff walk animation (457 KB)
walk-bb8-01.mov	Pixel art BB-8 rolling animation (417 KB)

Test plan

• Build and run — Bruce & Jazz appear as default (no behaviour change)
• Open Characters menu → switch to "Droids" → three pixel art droids appear on dock
• Switch back to "Bruce & Jazz" → original characters return
• Quit and relaunch → last selected pack is remembered
• Click a droid → popover opens with Claude terminal
• Thinking/completion bubbles show pack-appropriate phrases
• All four visual themes still work with both packs

[mpearon]

Would love to see a "Clippy & Co." back here too!

[adityasaravana]

oh man please merge this @ryanstephen