fix(prometheus): enforce path constraints and atomic write protocol (#1414)

* fix(prometheus): enforce path constraints and atomic write protocol

- Add FORBIDDEN PATHS section blocking docs/, plan/, plans/ directories
- Add SINGLE ATOMIC WRITE protocol to prevent content loss from multiple writes
- Simplify PROMETHEUS_AGENTS array to single PROMETHEUS_AGENT string

* fix: reconcile Edit tool signature in interview-mode.ts with identity-constraints.ts

Identified by cubic: Edit tool usage was inconsistent between files.

- interview-mode.ts showed: Edit(path, content)
- identity-constraints.ts showed: Edit(path, oldString="...", newString="...")

Updated interview-mode.ts to use the correct Edit signature with oldString and newString parameters to match the actual tool API and prevent agent hallucination.

src/agents/prometheus/interview-mode.ts

@@ -323,7 +323,7 @@ Write(".sisyphus/drafts/{topic-slug}.md", initialDraftContent)
 **Every Subsequent Response**: Append/update draft with new information.
 \`\`\`typescript
 // After each meaningful user response or research result
-Edit(".sisyphus/drafts/{topic-slug}.md", updatedContent)
+Edit(".sisyphus/drafts/{topic-slug}.md", oldString="---\n## Previous Section", newString="---\n## Previous Section\n\n## New Section\n...")
 \`\`\`
 
 **Inform User**: Mention draft existence so they can review.