docs(examples): add explain_decision example [skip-runtime-e2e] (#185)

* docs(examples): add explain_decision example end-to-end

Closes the example-parity gap: client.explain_decision() shipped in
April but no runnable example existed. Mirror the Rust SDK pattern
(axonflow-sdk-rust#29) so callers can copy a complete demo without
reading the SDK source.

Reads AXONFLOW_DECISION_ID from env (typically harvested from a
recent block response or audit row), calls explain_decision, and
prints every ADR-043 field — policy matches, matched rules, override
availability, historical hit count.

Signed-off-by: Saurabh Jain <saurabhjain1592@gmail.com>

* fix(lint): apply ruff format to examples/explain_decision.py

Signed-off-by: Saurabh Jain <saurabhjain1592@gmail.com>

---------

Signed-off-by: Saurabh Jain <saurabhjain1592@gmail.com>

Author: saurabhjain1592

examples/explain_decision.py

@@ -0,0 +1,88 @@
+"""Example: explain a previously-made AxonFlow policy decision.
+
+Implements the ADR-043 explainability flow. Given a decision_id (typically
+surfaced on the response of a blocked governed call, an audit_logs row, or
+the ``explain_decision`` MCP tool), this example fetches the structured
+explanation and renders the matched policies, risk level, and override
+availability.
+
+Required env vars:
+
+* ``AXONFLOW_AGENT_URL``       (default: http://localhost:8080)
+* ``AXONFLOW_CLIENT_ID``       (default: community)
+* ``AXONFLOW_CLIENT_SECRET``   (default: empty)
+* ``AXONFLOW_DECISION_ID``     the decision to explain
+
+Get a decision_id quickly by hitting a known-blocked policy::
+
+    curl -u "$AXONFLOW_CLIENT_ID:$AXONFLOW_CLIENT_SECRET" \\
+         -X POST $AXONFLOW_AGENT_URL/api/v1/mcp/check-input \\
+         -H 'Content-Type: application/json' \\
+         -d '{"connector_type":"postgres","operation":"execute",
+              "statement":"SELECT 1; DROP TABLE users;--","user_token":"u1"}'
+
+then read decision_id from the block response or the most recent audit row.
+"""
+
+import asyncio
+import os
+import sys
+
+from axonflow import AxonFlow
+
+
+async def main() -> None:
+    decision_id = os.environ.get("AXONFLOW_DECISION_ID", "")
+    if not decision_id:
+        print(
+            "AXONFLOW_DECISION_ID must be set (a decision_id from a recent blocked call)",
+            file=sys.stderr,
+        )
+        sys.exit(2)
+
+    endpoint = os.environ.get("AXONFLOW_AGENT_URL", "http://localhost:8080")
+    print(f"Initializing AxonFlow client at {endpoint}...")
+    async with AxonFlow(
+        endpoint=endpoint,
+        client_id=os.environ.get("AXONFLOW_CLIENT_ID", "community"),
+        client_secret=os.environ.get("AXONFLOW_CLIENT_SECRET", ""),
+    ) as client:
+        print(f"Explaining decision {decision_id}...\n")
+        exp = await client.explain_decision(decision_id)
+
+        print("=== Decision Explanation ===")
+        print(f"  decision_id: {exp.decision_id}")
+        print(f"  timestamp:   {exp.timestamp.isoformat()}")
+        print(f"  decision:    {exp.decision}")
+        print(f"  reason:      {exp.reason}")
+        if exp.risk_level:
+            print(f"  risk_level:  {exp.risk_level}")
+        if exp.tool_signature:
+            print(f"  tool:        {exp.tool_signature}")
+
+        print(f"\n  policy_matches ({len(exp.policy_matches)}):")
+        for i, m in enumerate(exp.policy_matches):
+            print(
+                f"    [{i}] {m.policy_id} ({m.policy_name or '(unnamed)'}) — "
+                f"action={m.action or '-'} risk={m.risk_level or '-'} "
+                f"allow_override={m.allow_override}"
+            )
+
+        if exp.matched_rules:
+            print(f"\n  matched_rules ({len(exp.matched_rules)}):")
+            for r in exp.matched_rules:
+                print(
+                    f"    {r.policy_id} on {r.rule_id or '(no rule id)'}: "
+                    f"matched={r.matched_on or '-'}"
+                )
+
+        print(f"\n  override_available:           {exp.override_available}")
+        if exp.override_existing_id:
+            print(f"  override_existing_id:         {exp.override_existing_id}")
+        print(f"  historical_hit_count_session: {exp.historical_hit_count_session}")
+        if exp.policy_source_link:
+            print(f"  policy_source_link:           {exp.policy_source_link}")
+
+
+if __name__ == "__main__":
+    asyncio.run(main())