feat(chatcut): add v6 Canvas plugin workflow

[smackgg]

What

• add Hook and convax.plugin/6 schema/tooling support to the public package registry
• add the ChatCut Canvas Plugin and Plugin-owned Skill
• add the reviewed ChatCut connected-media import companion

Why

ChatCut should work as a normal Convax Plugin: media connects to its Canvas node, explicit import uses the verified companion, and Agent editing uses ChatCut's standard remote MCP/OAuth flow. Concrete integration code stays in convax-plugins; the host remains provider-neutral.

Dependency

• host support merged in microvoid/convax#45

Checks

• bun check
• 22 packages validated
• workspace typechecks and tests
• companion builds
• 316 tests passed across workspace and registry suites
• deterministic pack and Registry/Showcase index builds