docs(agent-context): clarify install and marker requirement

- README: clarify install command must be run from an initialized Spec Kit project root
- config: correct context_markers requirement from REQUIRED to OPTIONAL

Author: yanukadeneth99

extensions/agent-context/README.md

@@ -17,7 +17,7 @@ Not every Spec Kit user wants Spec Kit to write into the coding agent's context
 
 ## Installation
 
-To install the extension, run the following command after installing Spec Kit.
+To install the extension, from the root of an initialized Spec Kit project, run:
 
 ```bash
 specify extension add agent-context

extensions/agent-context/agent-context-config.yml

@@ -14,7 +14,7 @@ context_file: ""
 context_files: []
 
 # WHAT: Markers (Delimiters) for the managed Spec Kit section. All information injected by this extension will only be done in between these markers.
-# REQUIREMENT: REQUIRED. Only change if you wish to have a custom marker name.
+# REQUIREMENT: OPTIONAL. Only change if you wish to have a custom marker name.
 # EXAMPLE:
 # context_markers:
 #   start: "<!-- AGENT SPEC KIT CONTEXT START -->"