Refactor ai.nix into ai/ directory (flakes-ojyn)

Convert mixins/home/host/common/ai.nix into a folder (ai/), moving scripts/installs/ under ai/installs/ and extracting inline scripts into bash files.

Summary of Changes

• Converted mixins/home/host/common/ai.nix → ai/ directory
• ai/default.nix: autowire pattern for extensibility
• ai/ai.nix: package declarations (bun, nodejs_25)
• ai/installs/default.nix: gatherScriptPackages_bash
• Moved scripts/installs/install-claude-mcp-context7.bash and install-claude-mcp-verena.bash → ai/installs/
• Extracted inline clone-beans and install-ccline scripts into ai/installs/
• Removed scripts/installs/ directory (now empty)