feat(jest): add ng-add schematic (#22)

[just-jeb]

PR Checklist

Please check if your PR fulfills the following requirements:

• Tests for the changes have been added (for bug fixes / features) — deferred (see Other information)
• Docs have been added / updated (for bug fixes / features) — README update can land alongside follow-up packages

PR Type

What kind of change does this PR introduce?

[ ] Bugfix
[x] Feature
[ ] Code style update (formatting, local variables)
[ ] Refactoring (no functional changes, no api changes)
[ ] Build related changes
[ ] CI related changes
[ ] Documentation content changes
[ ] Other... Please describe:

What is the current behavior?

Users must manually configure @angular-builders/jest by editing angular.json, swapping the karma builder for @angular-builders/jest:run, removing karma/jasmine devDependencies, deleting karma.conf.js / src/test.ts, and updating tsconfig.spec.json types. There is no ng add support.

Issue Number: #22

What is the new behavior?

Running ng add @angular-builders/jest now:

1. Rewrites architect.test.builder to @angular-builders/jest:run (clears karma-specific options when the previous builder was karma).
2. Removes the standard karma/jasmine devDependencies (karma, karma-chrome-launcher, karma-coverage, karma-jasmine, karma-jasmine-html-reporter, jasmine-core, @types/jasmine).
3. Adds @angular-builders/jest to devDependencies pinned to its own version.
4. Deletes karma.conf.js and src/test.ts when present.
5. Updates tsconfig.spec.json so types swaps jasmine → jest and the files: ["src/test.ts"] entry is dropped (textual swap so JSONC comments survive).
6. Schedules a NodePackageInstallTask (skippable via --skip-install).

The Rule is exported as default from dist/schematics/ng-add/index.js, and package.json declares "schematics": "./dist/schematics/collection.json".

Does this PR introduce a breaking change?

[ ] Yes
[x] No

Other information

Scope: intentionally narrow — @angular-builders/jest only, ng-add only. Other packages and version migrations are follow-up PRs.

Structural lessons from the closed #2229: that PR had four blockers, all addressed here:

Issue in #2229	Fix in this PR
Schematics TS sources never compiled to dist/ (not in any tsconfig include)	New tsconfig.schematics.json (rootDir src/schematics, outDir dist/schematics) wired into the package build script alongside the existing tsc -p tsconfig.lib.json.
collection.json referenced paths like ./schematics/ng-add/index that don't resolve after compile	collection.json lives in src/schematics/ and is copied via copy:schematics; factory path is ./ng-add/index#default, correct relative to the dist'd collection.json.
Cross-package source-tree imports (../../../common/schematics/...)	Rule uses only @angular-devkit/schematics + @angular-devkit/core (already direct deps). No source-tree imports across packages.
No test coverage committed	A SchematicTestRunner-based spec is staged out in code comments. See "Test deferral" below.

Test deferral: @angular-devkit/schematics/testing transitively imports ora@9 (ESM-only). The repo's current ts-jest config (used for unit tests) rejects it with "Cannot use import statement outside a module". A follow-up PR will adjust transformIgnorePatterns / moduleNameMapper and add the spec — the assertions it should cover are enumerated in a TODO at the top of ng-add/index.ts.

Verification: yarn workspace @angular-builders/jest build produces:

• dist/schematics/collection.json
• dist/schematics/ng-add/index.js (default export = Rule factory, verified via require())
• dist/schematics/ng-add/schema.json

and the existing 35 unit tests still pass.

Follow-ups (separate PRs):

• Add ng-add schematics for @angular-builders/custom-webpack and @angular-builders/custom-esbuild following the same structural pattern.
• Add the SchematicTestRunner unit spec for jest ng-add (requires Jest ESM transform config fix).
• Version migrations (e.g. v18 → v21) — out of scope here.