build: Add root-level `build:all` / `test:all` npm scripts

Summary

The root package.json currently exposes only three
scripts (doctor, prepare, update-schemas). There is no single command
to build and test the whole monorepo. Contributors evaluating a PR must
manually chain per-workspace commands and remember the correct ordering
(utils must build first).

This issue proposes adding two convenience aggregate scripts to the root
package.json so a full local check mirrors what CI does.

Motivation

• Discoverability: A newcomer can run one command instead of learning the
  workspace build/test ordering.
• Correctness: Encodes the "build utils first" rule so downstream
  workspaces (functions, frontend) pick up compiled output in
  utils/dist/.
• PR evaluation: Gives a canonical "full test" invocation.

Proposed changes

Add the following entries to the scripts block of the root package.json:

"build:all": "npm run build -w utils && npm run build -w functions && npm run build -w frontend",
"test:all": "npm test --workspaces --if-present"

Rationale per script

Full-test invocation

The intended "run everything" command becomes:

npm ci && npm run build:all && npm run test:all

Caveats / things to verify

[!IMPORTANT]
Java 21 requirement. The functions workspace's test script is
test:unit && test:firestore, and test:firestore launches the Firebase
emulator, which requires Java 21. Therefore test:all will fail on a
machine without Java 21. This is intentional: a full test run mirrors CI.
Contributors who need to scope down (e.g. a PR that doesn't touch
functions) can run individual workspace commands directly, e.g.
npm run test:unit -w functions.

• The three workspaces (utils, functions, frontend) each currently
  define both build and test scripts, so --if-present on test:all is
  belt-and-suspenders but future-proof.
• Tests strictly only need utils built (they import
  @deliberation-lab/utils from dist/); functions/frontend compile on
  the fly under jest. build:all is intentionally broader so it also catches
  build/compile errors during a PR eval.
• Consider whether the documented workflow in the root AGENTS.md
  ("Testing" section) should be updated to reference these new scripts.

Acceptance criteria

• build:all and test:all added to root package.json.
• npm ci && npm run build:all && npm run test:all succeeds on a machine
  with Java 21.
• Root AGENTS.md "Testing" section updated to mention the new scripts
  (optional but recommended).

Edit the scripts block in the root package.json, and review the Testing section of AGENTS.md for the optional documentation update. Start by inspecting the existing workspace scripts, then run npm ci followed by npm run build:all and npm run test:all on a machine with Java 21. Done means both aggregate scripts exist and the full command succeeds.