feat(3597): split test suites and add Node 22/24/26 OS matrix

scripts/run-tests.cjs gains `--suite <name>` filtering using a filename
suffix convention (`*.security.test.cjs`, `*.integration.test.cjs`, …).
Files with no marker are `unit` (the default fast lane); files with a
marker land in the matching suite. No `--suite` flag preserves the prior
behavior of running every test (backcompat for `npm test` and
`npm run test:coverage`).

New package scripts wire the suites to stable entrypoints:
test:unit, test:integration, test:install, test:security, test:slow,
test:coverage:unit, test:coverage:all. Unknown suite → exit 2 with the
list of valid suites; empty suite → exit 0 with a stderr notice so empty
lanes (e.g. `security` before adversarial tests land) don't gate CI.

CI matrix grows from `ubuntu × {22,24}` + a single macOS lane to
`{ubuntu, macos, windows} × {22, 24, 26}`. `fail-fast: false` so one
lane failure doesn't cancel siblings. Node 26 is `continue-on-error`
until actions/setup-node stabilises that image. PR CI runs unit +
integration + security on every cell; `install` and `slow` only on
`main` push. A dedicated `coverage` job runs `test:coverage:unit` on
ubuntu/Node 24 and uploads the report.

Grouping policy lives in docs/TESTING-SUITES.md with a pointer from
CONTRIBUTING.md. New harness test covers arg parsing, filter selection,
empty-suite behavior, and failure propagation.

Closes #3597.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

package.json

@@ -76,6 +76,13 @@
     "changeset": "node scripts/changeset/new.cjs",
     "changelog:render": "node scripts/changeset/cli.cjs render",
     "test": "node scripts/run-tests.cjs",
-    "test:coverage": "c8 --check-coverage --lines 70 --reporter text --include 'get-shit-done/bin/lib/*.cjs' --exclude 'tests/**' --all node scripts/run-tests.cjs"
+    "test:unit": "node scripts/run-tests.cjs --suite unit",
+    "test:integration": "node scripts/run-tests.cjs --suite integration",
+    "test:install": "node scripts/run-tests.cjs --suite install",
+    "test:security": "node scripts/run-tests.cjs --suite security",
+    "test:slow": "node scripts/run-tests.cjs --suite slow",
+    "test:coverage": "c8 --check-coverage --lines 70 --reporter text --include 'get-shit-done/bin/lib/*.cjs' --exclude 'tests/**' --all node scripts/run-tests.cjs",
+    "test:coverage:unit": "c8 --check-coverage --lines 70 --reporter text --include 'get-shit-done/bin/lib/*.cjs' --exclude 'tests/**' --all node scripts/run-tests.cjs --suite unit",
+    "test:coverage:all": "npm run test:coverage"
   }
 }