restore lint by pinning typescript below 7 (#1131)

#1090 bumped typescript to 7.0.2. typescript-eslint declares
`typescript: >=4.8.4 <6.1.0`, and TypeScript 7 removed `ts.Extension`, which
@typescript-eslint/typescript-estree dereferences at import time:

    node_modules/@typescript-eslint/typescript-estree/dist/create-program/shared.js:59
        ts.Extension.Cjs,
    TypeError: Cannot read properties of undefined (reading 'Cjs')

ESLint hits that while loading eslint.config.mjs, so `npm run lint` fails before
linting anything. Node reports it as ERR_INTERNAL_ASSERTION, which hides the
cause. Every open PR fails this check, not just new ones.

Even the latest typescript-eslint prerelease still caps at <6.1.0, so there is
nothing to upgrade to yet. Pin back to ~6.0.3 and tell dependabot to hold major
typescript bumps until the ecosystem catches up.

Also fixes biome.json pointing vcs.defaultBranch at dev-2.5.0, a branch that no
longer exists.

biome.json

@@ -4,7 +4,7 @@
     "enabled": true,
     "clientKind": "git",
     "useIgnoreFile": true,
-    "defaultBranch": "dev-2.5.0"
+    "defaultBranch": "dev-2.6.1"
   },
   "files": {
     "ignoreUnknown": true,