ci: enable grouped dependabot updates for npm (#391)

## Summary

There is no `.github/dependabot.yml` today, so dependabot runs
**security updates only**, and those bump one package at a time. Astro
releases `astro` and `@astrojs/*` in lockstep with narrow peer ranges,
so a single-package bump inside that family frequently cannot resolve at
all: #384 fails `npm ci` with ERESOLVE, as did #337 before it.

This enables npm version updates with three groups:

- `astro` / `astro-security` — `astro` + `@astrojs/*` in one PR, so
dependabot resolves the family in a single pass. This is the fix for the
#384 class.
- `rest` — everything else in one PR, so the weekly run stays at two
PRs.

## What this does not fix

Grouping fixes resolution, not code. A Starlight minor is a breaking
release, so a grouped PR still fails the build check whenever the new
version needs source changes (0.39 changed the `autogenerate` sidebar
shape, 0.42 rewrote the mobile-menu markup). Those still get taken over
by hand on an `infra/` branch, which is also the only way to get a
preview: `preview-deployment.yml` is skipped on dependabot PRs because
their token is read-only.

`.agents/upstream-tracking.md` gains a section recording that, plus the
two lockfile and rendering checks worth running when taking a bump over.

Author: marc0olo

.agents/upstream-tracking.md

@@ -235,3 +235,41 @@ If a shallow clone cannot resolve a pinned commit:
 git -C .sources/<repo> fetch --unshallow
 git -C .sources/<repo> checkout <commit>
 ```
+
+## Build dependencies (npm)
+
+The packages in `package.json` are a separate axis: they decide whether the site
+builds, not whether its content is accurate. `.github/dependabot.yml` keeps them
+current with weekly version updates plus security updates.
+
+`astro` and `@astrojs/*` are grouped into a single PR because Astro releases them
+in lockstep and pins their peer ranges narrowly. A bump to one of them alone often
+cannot resolve at all: astro 7.2.10, for example, moved its
+`@astrojs/markdown-remark` peer from an exact pin to `^7.3.0`, which only
+`@astrojs/mdx` 8 satisfies, which only ships with Starlight 0.42. Grouping lets
+dependabot resolve the whole family in one pass.
+
+Grouping a security update coalesces only the family members that each carry an
+advisory, so an advisory against `astro` alone still arrives as a one-package PR
+that cannot resolve. The weekly version update is what keeps that rare, by
+leaving little room between the family's releases and what is committed.
+
+Grouping fixes the resolution, not the code. A Starlight minor is a breaking
+release, so a grouped PR still fails the build check whenever the new version
+needs source changes (0.39 changed the `autogenerate` sidebar shape; 0.42
+rewrote the mobile-menu markup). Take those over by hand on an `infra/` branch:
+`preview-deployment.yml` is skipped on dependabot PRs, whose token is read-only,
+so a bump with visual impact needs a maintainer branch to get a preview at all.
+
+Two things to check when taking one over:
+
+- Regenerating `package-lock.json` on macOS prunes what does not apply locally:
+  the `libc` fields on the Linux binding packages, and, when `node_modules` is
+  present, the top-level `@emnapi/*` packages that `npm ci` needs on Linux. Move
+  `node_modules` aside, regenerate with `npm install --package-lock-only`, copy
+  the `libc` fields back from the previous lockfile, then validate with `npm ci`,
+  which reads the lock without rewriting it.
+- `npm ci && npm run build` is the gate, but it exits 0 on rendering
+  regressions. Diff `dist/` against a `main` baseline: `llms.txt`,
+  `llms-full.txt`, `sitemap.xml` and the `.md` endpoints should be identical,
+  and every HTML difference should trace to a documented upstream change.

.github/dependabot.yml

@@ -0,0 +1,40 @@
+version: 2
+
+updates:
+  # Version updates are enabled only so that lockstep families can be bumped
+  # together. Without them, dependabot runs security updates only, and those
+  # touch one package at a time: an astro security bump alone cannot resolve,
+  # because astro pins its @astrojs/* peers to a narrow range (see #384).
+  - package-ecosystem: npm
+    directory: "/"
+    schedule:
+      interval: weekly
+      day: monday
+    open-pull-requests-limit: 5
+    commit-message:
+      prefix: "chore(deps)"
+    groups:
+      # astro and @astrojs/* are released in lockstep; one PR per family so
+      # dependabot resolves them in a single pass.
+      astro:
+        applies-to: version-updates
+        patterns:
+          - "astro"
+          - "@astrojs/*"
+      # Only coalesces family members that each carry an advisory, so a single
+      # astro advisory still arrives as a one-package PR that cannot resolve.
+      # What keeps that rare is the weekly version update above; when one does
+      # show up, take it over by hand.
+      astro-security:
+        applies-to: security-updates
+        patterns:
+          - "astro"
+          - "@astrojs/*"
+      # Everything else in one PR, to keep the weekly run to two PRs.
+      rest:
+        applies-to: version-updates
+        patterns:
+          - "*"
+        exclude-patterns:
+          - "astro"
+          - "@astrojs/*"