refactor: stop re-exporting components into the app tree

[aklkv]

Stacked on #988 (on #987, on #986): merge those first, then retarget this to main.

The build re-exported every component, icon (2,760) and modifier into the consuming app's tree (ember-addon.app-js, about 3,000 entries), so that loose-mode .hbs templates could resolve them by name, such as <Carbon::TextInput>. Nothing documented that:

• the README, the Storybook intro and the agent skill all import from the barrels;
• the repo's tests and Storybook import directly too;
• src/template-registry.ts, which would have given loose-mode use its types, was still the empty blueprint stub.

The map also made up nearly all of package.json's 346 KB.

Changes

• App tree: only the three services stay in it (rollup.config.mjs), because the components look them up by name (@service('carbon.dialog-manager'), carbon.notifications, carbon.ai-chat-session). package.json's app-js map drops to three entries, and the file to about 6 KB.

• Removed: the empty src/template-registry.ts, and src/components/index.ts, which only kept carbon-components-ember/components/index working. The package's "." export and the empty src/index.ts stay as the blueprint has them.

• Docs: the README says templates must be strict mode, in a callout under Use:

  Strict mode only. Import components, icons and modifiers as above. Loose-mode .hbs templates can't resolve them by name: <Carbon::TextInput> and {{component "carbon/text-input"}} don't work. Write the templates that use them as .gts or .gjs.

  Its install note now says classic ember-cli apps need ember-template-imports. The Storybook intro and the agent skill say the same, and AGENTS.md says not to refer to components by name.

• Tag's console error no longer calls it Carbon::Tag.

Breaking

• Loose-mode .hbs templates can't resolve the addon's components, icons or modifiers by name. Import them into .gts or .gjs templates.
• carbon-components-ember/template-registry and carbon-components-ember/components/index are gone. Import from carbon-components-ember/components.

Verified

• A clean build puts only the three services in dist/_app_.
• publint and the type-check pass, and the generated barrels are unchanged.
• The repo's tests and Storybook import source paths, so they don't depend on the app tree.

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