docs: synchronize documentation with current implementation

- Root README.md: proper project overview with quick start
- Root AGENTS.md: add MAILHOG_SMTP_PORT, update env config note
- demo/README.md: add MailHog SMTP port (4019) to service table
- demo/scripts/validate-all.sh: fall back to demo.env.template
  when demo.env not present, add MAILHOG_SMTP_PORT to required vars,
  mask variable values in validation output

💘 Generated with Crush

Assisted-by: GLM-5.1 via Crush <crush@charm.land>

AGENTS.md

@@ -139,7 +139,7 @@ docker run --rm -v "$(pwd):/workdir" hadolint/hadolint <path-to-dockerfile>
    - ArchiveBox (4013) - Web archiving solution
    - Tube Archivist (4014) - YouTube video archiving (requires ta-redis + ta-elasticsearch)
    - Wakapi (4015) - Open-source WakaTime alternative (time tracking)
-   - MailHog (4017) - Web and API based SMTP testing
+   - MailHog (4017 Web, 4019 SMTP) - Web and API based SMTP testing
    - Atuin (4018) - Magical shell history synchronization
 
 5. **Companion Services** (internal only, no host ports)
@@ -147,7 +147,7 @@ docker run --rm -v "$(pwd):/workdir" hadolint/hadolint <path-to-dockerfile>
    - ta-elasticsearch - Elasticsearch index for Tube Archivist
 
 ### Configuration Management
-- **Environment Variables**: All configuration via `demo/demo.env`
+- **Environment Variables**: All configuration via `demo/demo.env` (copy from `demo/demo.env.template`)
 - **Template-Based**: `docker-compose.yml` generated from `docker-compose.yml.template` using `envsubst`
 - **Dynamic User Detection**: UID/GID automatically detected and applied
 - **Service Discovery**: Automatic via Homepage labels in docker-compose.yml
@@ -342,6 +342,7 @@ ARCHIVEBOX_PORT=4013
 TUBE_ARCHIVIST_PORT=4014
 WAKAPI_PORT=4015
 MAILHOG_PORT=4017
+MAILHOG_SMTP_PORT=4019
 ATUIN_PORT=4018
 
 # Demo Credentials (NOT FOR PRODUCTION)