## Why
The initial research pass focused on agent definitions. An audit identified
critical and minor gaps in both doc sets around plugin publishing, marketplace
registration, and the Copilot extensibility model.
## Implementation Notes
Claude Code gaps filled: end-to-end publish walkthrough (scaffold → validate →
tag → host → register), CLI vs in-session command surface equivalence, all six
marketplace source URL formats, plugin update/upgrade lifecycle, interactive
plugin manager UI, private marketplace auth, `commands` vs `skills/` distinction.
Copilot gaps filled: discovered that the GitHub App-based Copilot Extensions
track was sunset November 2025. Created copilot-extensions.md as historical
reference (deprecated, with MCP servers as the current replacement path).
Documented the agent vs. skillset extension type distinction, OAuth install
flow, and VS Code Chat Participants as the surviving @mention mechanism.
overview.md updated with a "Which Track to Use" decision table covering all
four active tracks plus the deprecated one.
Co-authored-by: Claude Sonnet 4.6 <noreply@anthropic.com>
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