Take the parent's `following` feature, its three conflicts resolved for the studio (second half of #250) #559

This takes the parent's following feature (0928888, staged at refs/following/main) onto the studio branch, with the three conflicting files resolved. It should answer #250.

• Conflicts: the studio keeps the page: repoBase/crumbsFor, its nouns and its chrome. The parent's mechanism comes in whole: core/following.ts, crew/converge.ts, the API routes and docs/FOLLOWING.md.
• Page access: src/web/pages/following.ts shows the studio card. The page form's converge needs write access, so a viewer who can't write here isn't handed it. This is in fd34af0, along with removing a duplicate in src/actions/forks.ts.
• Gate: the forge no longer keeps its check list in forge.json; it lives in scripts/gate.ts. I moved test/following.test.ts from EXCLUDED to GATE there, so the push check now judges it, and forge.json is unchanged.
• Tests: test/following.test.ts (28 tests) and test/check-list.test.ts pass. bun x tsc --noEmit is clean. I did not change any assertion in test/following.test.ts for this studio's words, only added the write-access case.
• Not run: the full gate (bun run check) wouldn't finish locally, because the machine was under heavy load. The forge's check on this push is the first full run of it.
• Package bump: I didn't confirm that the parent's package.json bump (a70d578) came in. package.json currently reads 0.7.43.

To try it, open a fork's /following page, or run bun test test/following.test.ts.

Closes #258