Retire the basecamp@37signals alias after the 7-day rename window #857

#840 renames the agent plugin to basecamp-cli@37signals. basecamp/plugins#8 keeps basecamp in the 37signals marketplace as a deprecated alias with the same source, for 7 days from the basecamp-cli release that ships #840 (decided 2026-10-05).

When the window ends:

1. claude-plugins: a PR removes the basecamp alias from .claude-plugin/marketplace.json and relaxes rule 2 of bin/check-marketplace (its header names this step).
2. basecamp-cli: a release stops treating basecamp@37signals as the old CLI plugin: ClaudeLegacyPluginKey in isStalePluginKey, CodexLegacyPluginKey, and the rename notice in agent_hook_rename.go (see AGENTS.md, "During the deprecation window").
3. Only after both: pointing basecamp@37signals at the hosted plugin is its own decision. Before step 2 ships, basecamp setup would uninstall it.

Set the due date when the release that ships #840 is cut.