feat(core): support dynamic variable substitution in system prompt override (#17042)

docs/cli/system-prompt.md

@@ -56,6 +56,38 @@ error with: `missing system prompt file '<path>'`.
 When `GEMINI_SYSTEM_MD` is active, the CLI shows a `|⌐■_■|` indicator in the UI
 to signal custom system‑prompt mode.
 
+## Variable Substitution
+
+When using a custom system prompt file, you can use the following variables to
+dynamically include built-in content:
+
+- `${AgentSkills}`: Injects a complete section (including header) of all
+  available agent skills.
+- `${SubAgents}`: Injects a complete section (including header) of available
+  sub-agents.
+- `${AvailableTools}`: Injects a bulleted list of all currently enabled tool
+  names.
+- Tool Name Variables: Injects the actual name of a tool using the pattern:
+  `${toolName}_ToolName` (e.g., `${write_file_ToolName}`,
+  `${run_shell_command_ToolName}`).
+
+  This pattern is generated dynamically for all available tools.
+
+### Example
+
+```markdown
+# Custom System Prompt
+
+You are a helpful assistant. ${AgentSkills}
+${SubAgents}
+
+## Tooling
+
+The following tools are available to you: ${AvailableTools}
+
+You can use ${write_file_ToolName} to save logs.
+```
+
 ## Export the default prompt (recommended)
 
 Before overriding, export the current default prompt so you can review required