Delete the empty installation configuration fields

[SaladDay]

GET /core/v1/installation returned configuration.path, apply_command and applied_at as "", "" and null every time. Core reports the process settings it loaded and never sets them, and their only producer was the parser deleted in #499. configuration itself was declared nullable, but Core always sets it and exits at startup if it can't build it.

• Core: InstallationConfiguration now holds only settings, and Installation.Configuration is a non-null value. core.openapi.yaml is regenerated with make openapi.
• agents-client: the type, the projection and the tests drop the three fields; configuration must be an object with only settings.
• Web (System › Startup settings):
  • The never-rendered config-file/apply-command line, the last-applied time and the "Core did not report startup settings." branch are deleted, along with their i18n keys, unused CSS and orphaned locale strings.
  • The section now opens with its existing "These are the settings this Core process loaded." note.
  • The e2e fixture and spec are updated.
• Docs: admin-api.md, docs/web/console-api-usage.md (and zh), apps/web/DESIGN.md and PRODUCT.md. The latter two dropped stale claims that Web shows a copyable config path and apply command.

No blind review: the change only deletes fields that are always empty or never null.

Checks:

• go build ./..., go vet ./services/core/..., the installation API and cmd/server tests.
• make openapi (left no diff), make check-docs, check-names.py, translation test.
• agents-client typecheck and tests; apps/web typecheck and the System, i18n, locale, InstallationNotice and SandboxSetupWizard unit tests.
• Playwright runs in CI.

---

^{Need help on this PR? Tag @codesmith-bot with what you need. Autofix is disabled.}