docs: update README.md for skill command accuracy

Updated the agentsync skill command documentation in README.md to include
supported --source and --json flags for install, update, and uninstall
subcommands. This ensures the README reflects the actual CLI capabilities
revealed during codebase review.

Changes:
- Added [--source <source>] and [--json] to skill install/update/uninstall examples.
- Verified all other claims in README against the implementation.
- Ensured no other files were modified.

Co-authored-by: yacosta738 <33158051+yacosta738@users.noreply.github.com>

Author: google-labs-jules[bot]

README.md

@@ -236,9 +236,9 @@ agentsync status
 agentsync doctor [--project-root <path>]
 
 # Manage skills
-agentsync skill install <skill-id>
-agentsync skill update <skill-id>
-agentsync skill uninstall <skill-id>
+agentsync skill install <skill-id> [--source <source>] [--json]
+agentsync skill update <skill-id> [--source <source>] [--json]
+agentsync skill uninstall <skill-id> [--json]
 ```
 
 ### Status