feat(coding-agent): add prompt template argument defaults

packages/coding-agent/docs/prompt-templates.md

@@ -64,10 +64,11 @@ Type `/` followed by the template name in the editor. Autocomplete shows availab
 
 ## Arguments
 
-Templates support positional arguments and simple slicing:
+Templates support positional arguments, defaults, and simple slicing:
 
 - `$1`, `$2`, ... positional args
 - `$@` or `$ARGUMENTS` for all args joined
+- `${1:-default}` uses arg 1 when present/non-empty, otherwise `default`
 - `${@:N}` for args from the Nth position (1-indexed)
 - `${@:N:L}` for `L` args starting at N
 
@@ -80,6 +81,12 @@ description: Create a component
 Create a React component named $1 with features: $@
 ```
 
+Default values are useful for optional arguments:
+
+```markdown
+Summarize the current state in ${1:-7} bullet points.
+```
+
 Usage: `/component Button "onClick handler" "disabled support"`
 
 ## Loading Rules