docs: remove extraneous content — pipeline internals moved to architecture, changelog removed

- features.md: remove SQL impl detail from tasks section, sw.js reference from PWA section,
  and entire "LLM Chat — Internal Pipeline" section (moved to architecture.md)
- architecture.md: add "LLM Pipeline Internals" section (intent routing, tool loop, duplicate
  guards, context window, research pipeline, image cache)
- development.md: remove site-specific NFS path from custom runner instructions
- Remove changelog.md (duplicates git history)
- README.md: remove changelog link

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

README.md

@@ -41,7 +41,6 @@ Go to **Settings → General** to pull an LLM model. `qwen3:8b` or `llama3.1:8b`
 | [SSO / OAuth](docs/sso-oauth.md) | OIDC setup for Authentik, Keycloak, and other providers |
 | [API Reference](docs/api-reference.md) | All REST API endpoints |
 | [Android App](docs/android-app.md) | Flutter companion app architecture and feature status |
-| [Changelog](docs/changelog.md) | Development history |
 
 ## License