cli: add feynman extensions to list project and package extensions

[Dünya Baradari (Dunya-8a)]

Summary

Adds a read-only feynman extensions command that lists:

• Project extensions — loose .ts/.js files under ./extensions
• From packages — installed Pi packages that declare pi.extensions

It mirrors feynman packages list. Enumeration reuses buildConnectorResources through a new summarizeExtensions() helper, so the CLI and the workbench stay in sync (single source of truth).

Also clarifies the feynman update help text: extensions are provided by packages and update with them; there is no separate --extensions flag.

Relates to #187.

Why

Addresses the discovery/help gap behind #187. feynman update --extensions failed with a bare "Unknown option" and there was no way to see what extensions are active or understand how they relate to packages. This adds the missing discovery surface and makes the packages↔extensions relationship explicit in help. (#189 separately improves the raw unknown-flag error message.)

Example

◆ Project extensions
  my-tool  extensions/my-tool.ts

◆ From packages
  pi-web-access  2 extensions  (npm:pi-web-access)
  Extensions are provided by packages and update with them: feynman update [package].

Empty projects render clean "No project extensions found" / "No installed packages provide extensions" states.

Changes

• src/workbench/package-resources.ts — new exported summarizeExtensions(workingDir).
• src/cli.ts — handleExtensionsCommand + dispatch wiring.
• metadata/commands.mjs — registered extensions command, added help line, clarified update description.
• tests/summarize-extensions.test.ts — covers project + package + nested-exclusion + empty cases.

Testing

• npm run typecheck — clean
• npm test — 587/587 pass (2 new)
• Drove feynman extensions end-to-end on populated and empty projects.

Notes

• Branched off upstream/main; independent of cli: hint at feynman help on unrecognized flags #189 (the --extensions error-hint fix). No file overlap, so the two can merge in either order.
• Read-only command; does not add any --extensions flag (extensions have no independent update lifecycle — they ride along with packages).

[vercel]

Dünya Baradari (@Dunya-8a) is attempting to deploy a commit to the Companion Team on Vercel.

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[Advait Paliwal (advaitpaliwal)]

Closing as rejected for product scope. Issue #187 is the narrower update/help defect; #192 already adds the unknown-option help hint and documents that extensions update with their packages. This PR adds a new generic extension-inventory command and workbench resource surface, which is broader than the proven research-loop need.