feat: add AGENTS.md for quick start and CI guidelines; update documentation for dcr add and dcr build commands

docs/commands/dcr-add.md

@@ -5,11 +5,15 @@ Add a dependency to `dcr.toml`.
 ## Usage
 
 ```bash
-dcr add <name> <source> [options]
+dcr add <name> [source] [options]
 ```
 
+If `<source>` is omitted and a registry is configured, DCR will look up the
+package by name in the registry and add it as a version dependency.
+
 ## Sources
 
+- *omitted*: Look up in the configured registry.
 - `path:./path/to/lib`: Local directory.
 - `github:user/repo`: GitHub repository.
 - `gitlab:user/repo`: GitLab repository.
@@ -29,4 +33,5 @@ dcr add my_lib path:./libs/my_lib
 dcr add cool_lib github:user/cool_lib
 dcr add shared_lib gitlab:org/shared --branch develop
 dcr add custom_lib git:git.internal.com/r/lib --tag v1.0.0
+dcr add mylib                    # Looks up in registry
 ```