Add pixel terminal skins and agent theme packs

[teo-nex]

Summary

• Add a fixed-geometry pixel-skin terminal renderer with six built-in themes (Sakura, Matrix, Forest Cabin, Gold & Black, Cat, Gothic Eclipse), each with minimal, detailed, and master artwork for idle, working, and completed states.
• Add theme backgrounds, on-frame controls, F4 master-detail behavior, and a custom theme pack creator/importer. A valid install ZIP contains the nine named terminal PNGs plus background.png.
• Add a local agent-control API for listing, installing, and selecting skin packs, with validation and persistence.
• Add in-app agent prompt, format documentation, and an offline kit: CanvasTTY design for agents (ZIP).
• Integrate with current main session placement, fullscreen, plugin-card actions, and lazy headless terminal loading.

Verification

• npm test: 1205 passed, 2 skipped, 0 failed.
• npm run build: passed.
• npm run audit:secrets: passed.
• git diff --check: passed.
• The published branch tree SHA matches the locally tested tree exactly (468ca794752cbb24dfb763ca6bbd325a3ecf277a).

Review note

The six built-in themes add about 82 MB of source assets. The example ZIP in the companion repository is an installable Gothic Eclipse theme; the agent kit ZIP is documentation/reference material and is not itself an installable theme.