Name Docs capabilities by their function

[brsbl]

Summary

• rename the BB-owned Docs thread-panel registration to Document
• rename the BB-owned Docs file-opener registration to Markdown
• verify plugin details render the real registered capability names without Docs-specific presentation logic

Verification

• 30 focused plugin-detail tests passed
• 57 Docs plugin tests passed
• 368 CLI tests passed
• Turbo typecheck passed for @bb/app, @bb/cli, and bb-plugin-simple-notes
• visually verified the enabled Docs detail page in the branch Electron app