feat(custom-webpack)!: remove Karma builder and tests (Angular 22 removes Karma)

[just-jeb]

PR Checklist

Please check if your PR fulfills the following requirements:

• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

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
[x] Documentation content changes
[x] Other... Please describe: Removal of a deprecated/unsupported builder ahead of Angular 22

What is the current behavior?

@angular-builders/custom-webpack ships a :karma builder that wraps
@angular-devkit/build-angular:karma. The package also carries Karma/Jasmine
example-app test fixtures (karma.conf.js / karma.conf.cjs, test targets in
angular.json, karma/jasmine devDependencies, puppeteer), Karma integration
tests, an e2e schema spec, and Karma documentation.

Issue Number: N/A

What is the new behavior?

The Karma builder and everything supporting it in custom-webpack are removed:

• Deleted src/karma/ (builder + schema extension), removed the karma entry
  from builders.json, the karma scheme from src/schemes.ts, and the
  export * from './karma' from src/index.ts.
• Deleted the e2e/custom-webpack-karma-schema.spec.ts schema test and the
  Karma documentation section from the package README.md.
• Removed the karma-specific .gitignore line.
• For the three custom-webpack example apps (sanity-app, sanity-app-esm,
  full-cycle-app): deleted karma.conf.*, removed the test (karma) target
  from angular.json, removed the karma/jasmine devDependencies plus
  puppeteer (these apps only used puppeteer for Karma), removed the karma
  test npm script, and removed Karma mentions from their READMEs.
• Removed the custom-webpack: Karma ... integration tests from
  packages/custom-webpack/tests/integration.js.
• Updated intent-layer docs (packages/custom-webpack/AGENTS.md,
  examples/AGENTS.md) to drop Karma references.

The custom-esbuild apps are untouched (they keep puppeteer and their unit-test
builder), and the CI PUPPETEER_SKIP_DOWNLOAD env / runner pinning from #2259 is
left as-is.

Verification: yarn install --immutable passes, yarn workspace @angular-builders/custom-webpack build succeeds (tsc + merge-schemes produce
only browser/server/dev-server/extract-i18n schemas, no karma), and
node scripts/discover-tests.js lists no custom-webpack: Karma tests.

Does this PR introduce a breaking change?

[x] Yes
[ ] No

The @angular-builders/custom-webpack:karma builder is removed. Angular 22 no
longer ships a Karma builder (@angular-devkit/build-angular:karma is gone), so
the wrapper cannot continue to exist. Users on a test target using
@angular-builders/custom-webpack:karma must migrate to a supported test runner
(Vitest / Web Test Runner, or @angular-builders/jest).

Other information

This is a v22-targeted breaking change to be merged only once Angular 22 is
released. Karma is removed upstream in Angular 22. A Vitest / Web Test Runner
replacement is tracked in #1928.

[just-jeb]

Holding this PR — Angular 22 does not remove Karma

Re-checked against the actual Angular 22.0.0-rc.3 install and Angular's published roadmap, and the premise behind this PR ("Angular 22 removes Karma") doesn't hold:

• The karma builder still ships in both @angular/build and @angular-devkit/build-angular in v22 — not flagged deprecated/hidden in builders.json. ng new flips the default runner to Vitest but still offers --test-runner karma; the Vitest unit-test builder is still [EXPERIMENTAL].
• ng update to v22 keeps existing Karma users on Karma (migrates the test builder to @angular/build:karma; does not delete karma.conf/test.ts or switch to Vitest). Vitest is opt-in via ng generate @angular/core:karma-to-vitest.
• Karma is deprecated, with security-only support continuing ~12 months after Vitest is marked stable (that clock hasn't started — still experimental). Angular's deprecation policy is a minimum of two majors, so full removal is ≈v24+, not v22.
• Our own v22 matrix confirms it: custom-webpack Karma+Chrome integration tests pass green on 22.0.0-rc.2.

Removing @angular-builders/custom-webpack's :karma builder in v22 would therefore strand Karma users that Angular itself still supports — a gratuitous breaking change, not a forced consequence.

Action: converting to draft and holding until the major where Angular actually removes Karma (re-evaluate at that cut). The v22 schematics design has been amended accordingly (custom-webpack ships ng-add only, no Karma-removal @22 migration; the v22 breaking set is now #2191 + #2212).

Sources: angular.dev/guide/testing/migrating-to-vitest · angular.dev/reference/releases · angular-cli commit 8654b3f.