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.

.github/dependabot.yml

@@ -37,6 +37,14 @@ updates:
       major-updates:
         update-types:
           - "major"
+    ignore:
+      # typescript-eslint declares `typescript: >=4.8.4 <6.1.0`, and TypeScript 7
+      # removed `ts.Extension`, which @typescript-eslint/typescript-estree reads
+      # at import time. Bumping to 7 makes `eslint .` fail to load its own config,
+      # so `npm run lint` cannot run at all. Drop this once typescript-eslint
+      # supports TypeScript 7.
+      - dependency-name: "typescript"
+        update-types: ["version-update:semver-major"]
 
   # Docker base images (docker/Dockerfile + docker-compose / compose-dev)
   - package-ecosystem: "docker"