fix: inject agent identity into system prompt for built-in agents (#2964)

Built-in agents (Sisyphus, Hephaestus, Atlas) now include explicit
identity section in their system prompts, matching custom agent behavior.

12 agent identity tests pass, 266 total pass, tsc clean.

Closes #2964

src/agents/dynamic-agent-prompt-builder.ts

@@ -8,6 +8,7 @@ export type {
 export { categorizeTools } from "./dynamic-agent-tool-categorization"
 
 export {
+  buildAgentIdentitySection,
   buildKeyTriggersSection,
   buildToolSelectionTable,
   buildExploreSection,