build: rename build:src and build:test to build:artifacts and build:check

build:artifacts produces every artifact, including the tsdown bundles,
the Desktop bundle, and Web, so naming it after sources undersold it;
build:check checks scripts, benchmarks, and website code besides tests.
The build.ts scopes, aggregate step ids, the Python builder flag
(--build-artifacts), workflows, callers, docs, and the build-split Agent
Note follow the new names. The note also records the naming
alternatives.

Refs #5852

AGENTS.md

@@ -92,9 +92,9 @@ pnpm run test:snapshot:record  # re-record expected outputs (needs key)
 pnpm run typecheck
 pnpm run lint
 pnpm run duplication    # cross-file TypeScript clone detection
-pnpm run build          # build:src + build:test, concurrently
-pnpm run build:src      # artifacts: tsc emits lib/types, tsdown bundles runtime, Web
-pnpm run build:test     # typecheck aggregate tests/scripts against current build:src output
+pnpm run build          # build:artifacts + build:check, concurrently
+pnpm run build:artifacts # artifacts: tsc emits lib/types, tsdown bundles runtime, Web
+pnpm run build:check    # typecheck aggregate tests/scripts against current build:artifacts output
 pnpm run hygiene        # publint + workspace/package/dependency checks + NodeNext consumer check
 pnpm run check:windows-wine  # ONLY when diagnosing a known Windows failure (needs wine); CI owns this signal
 pnpm run doc-sync       # documentation gates (scripts/run-gates.ts)
@@ -102,7 +102,7 @@ pnpm run test:docs      # quick documentation checks (no build; doc-quick aggreg
 pnpm run website:build  # VitePress build (doubles as dead-link check)
 pnpm dsh --profile headless "task"  # run one task from source (needs DEEPSEEK_API_KEY)
 pnpm run demo:ptc -- "task"  # headless PTC mode run (needs key)
-pnpm run dev:web | dev:desktop  # build:src, then launch; Web also rebuilds client bundles on edits. start:web | start:desktop skip the build
+pnpm run dev:web | dev:desktop  # build:artifacts, then launch; Web also rebuilds client bundles on edits. start:web | start:desktop skip the build
 make web|dev-web|desktop|dev-desktop|build  # the same commands; ARGS='--no-open' forwards options
 ```
 
@@ -117,7 +117,7 @@ Before pushing, follow [dsh-pre-push-checks](.agents/skills/dsh-pre-push-checks/
 - Match evidence to the surface: focused behavior tests, model/user-output snapshots, `doc-sync` for docs, built smokes for published paths, and real-API e2e for providers.
 - Never default to the full suite or repeat a passing check for commit or push. CI owns exhaustive coverage and the platform matrix; rehearse all locally only by explicit request, for CI diagnosis, or for an irreducibly repository-wide change.
 - `test:coverage`, not `test`, is the CI coverage gate ([why](docs/testing.md)).
-- Develop with `build:src`; before claiming completion, run `build:test` after the latest `build:src`, or `build` ([layout](docs/development.md#typescript-project-layout)).
+- Develop with `build:artifacts`; before claiming completion, run `build:check` after the latest `build:artifacts`, or `build` ([layout](docs/development.md#typescript-project-layout)).
 - **Web browser automation and GIF recording:** launch with `pnpm dsh web --patch apps/web/tests/pin-browse-picker.overlay.yml` to use the [in-page directory picker](apps/web/tests/pin-browse-picker.overlay.yml); omit this override only when testing native picker behavior explicitly.
 
 ## Secrets / .env