refactor(athena): split council-runtime-guidance into per-intent files under guidance/

Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-opencode)

Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>

src/agents/athena/guidance/explain.ts

@@ -0,0 +1,33 @@
+export const EXPLAIN_GUIDANCE = `
+<runtime_synthesis_rules>
+Use EXPLAIN synthesis.
+- Start with thesis.
+- Then mechanisms/interactions.
+- Then uncertainties and confidence.
+- Include a concise "why this matters" section tied to the user question.
+</runtime_synthesis_rules>
+
+<runtime_action_paths>
+Path type: INFORMATIONAL.
+
+1) Ask what to do with the explanation:
+Question({
+  questions: [{
+    question: "What should we do with this explanation?",
+    header: "Explanation Next Step",
+    options: [
+      { label: "Convert to action plan (Prometheus)", description: "Turn insights into a phased plan" },
+      { label: "Write to document", description: "Save to .sisyphus/athena/notes/ (named after this council session)" },
+      { label: "Ask follow-up", description: "Ask another explanatory question" },
+      { label: "Done", description: "No further action needed" }
+    ],
+    multiple: false
+  }]
+})
+
+2) Execute selected action:
+- Convert to action plan (Prometheus) -> switch_agent(agent="prometheus") with synthesized explanation and target outcome.
+- Write to document -> write the document to the ".sisyphus/athena/notes/" directory using the council session name from the council_finalize archive_dir, then report the exact path.
+- Ask follow-up -> ask user then restart the council workflow from Step 3 (intent classification).
+- Done -> acknowledge and end.
+</runtime_action_paths>`