Local file-based prompts don't support argument substitution despite CLI accepting arguments

The `/prompts` command accepts arguments for local .md file prompts, but the arguments are ignored and not substituted into the prompt content.

Current Behavior:
• CLI accepts: @my-prompt arg1 arg2 or /prompts get my-prompt arg1 arg2
• Arguments are passed to the function but unused
• Content from .md files is copied wholesale without substitution (line 1342 in prompts.rs)
• No placeholder syntax documented for local prompts

Expected Behavior:
• Arguments should be substituted into local prompt content
• Clear documentation of placeholder syntax (e.g., {{arg1}}, {arg1}, etc.)
• Support for required/optional arguments like MCP prompts

Context:
• MCP server prompts fully support arguments with validation
• Local prompts only work as static templates
• Feature gap between MCP and local prompt capabilities

Code Reference:
crates/chat-cli/src/cli/chat/cli/prompts.rs:1342 - content copied without processing

This creates confusion as the interface suggests argument support exists but it's not implemented for local file-based prompts.