fix(plan): use structured alternatives objects in plan explain output per spec

Convert alternatives_considered list of strings to structured objects with
index (1-based), description, and chosen fields in _build_explain_dict().
Rename output field from alternatives_considered to alternatives.
Update BDD tests in plan_explain.feature, plan_explain_cli_coverage.feature,
and plan_explain_steps.py to validate the new structured format.

Closes #9166

features/plan_explain.feature

@@ -13,7 +13,7 @@ Feature: Plan explain and decision tree CLI commands
     And the explain dict should contain key "question"
     And the explain dict should contain key "chosen"
     And the explain dict should contain key "type"
-    And the explain dict should contain key "alternatives_considered"
+    And the explain dict should contain key "alternatives"
     And the explain dict should not contain key "rationale"
     And the explain dict should not contain key "context_snapshot"
 
@@ -45,8 +45,10 @@ Feature: Plan explain and decision tree CLI commands
   Scenario: Explain includes alternatives by default
     Given a test decision with alternatives for explain
     When I build the explain dict with default options
-    Then the explain dict should contain key "alternatives_considered"
+    Then the explain dict should contain key "alternatives"
     And the alternatives list should have 2 items
+    And each alternative should have keys "index", "description", and "chosen"
+    And exactly one alternative should have chosen set to true
 
   # ------------------------------------------------------------------
   # plan explain - json format

features/plan_explain_cli_coverage.feature

@@ -49,7 +49,7 @@ Feature: Plan explain and tree CLI command coverage
     Given pec a mock DecisionService returning a decision with alternatives
     When pec I invoke "explain" with the decision id
     Then pec the exit code should be 0
-    And pec the output should contain "alternatives_considered"
+    And pec the output should contain "alternatives"
 
   # ------------------------------------------------------------------
   # tree_decisions_cmd - rich tree format

features/steps/plan_explain_steps.py

@@ -322,10 +322,27 @@ def step_snapshot_has_key(context: Context, key: str) -> None:
 
 @then("the alternatives list should have {count:d} items")
 def step_alternatives_count(context: Context, count: int) -> None:
-    alts = context.pe_explain_dict["alternatives_considered"]
+    alts = context.pe_explain_dict["alternatives"]
     assert len(alts) == count, f"Expected {count} alternatives, got {len(alts)}"
 
 
+@then('each alternative should have keys "index", "description", and "chosen"')
+def step_alternatives_have_required_keys(context: Context) -> None:
+    alts = context.pe_explain_dict["alternatives"]
+    for i, alt in enumerate(alts):
+        assert isinstance(alt, dict), f"Alternative {i} is not a dict: {alt!r}"
+        assert "index" in alt, f"Alternative {i} missing 'index' key: {list(alt.keys())}"
+        assert "description" in alt, f"Alternative {i} missing 'description' key: {list(alt.keys())}"
+        assert "chosen" in alt, f"Alternative {i} missing 'chosen' key: {list(alt.keys())}"
+
+
+@then("exactly one alternative should have chosen set to true")
+def step_exactly_one_chosen(context: Context) -> None:
+    alts = context.pe_explain_dict["alternatives"]
+    chosen_count = sum(1 for alt in alts if alt.get("chosen") is True)
+    assert chosen_count == 1, f"Expected exactly 1 chosen alternative, got {chosen_count}"
+
+
 @then('the json output should contain "{text}"')
 def step_json_contains(context: Context, text: str) -> None:
     assert text in context.pe_json_output, f"Expected '{text}' in JSON output"

src/cleveragents/cli/commands/plan.py

@@ -3909,7 +3909,14 @@ def _build_explain_dict(
         "is_correction": decision.is_correction,
         "superseded": decision.is_superseded,
         "created_at": decision.created_at.isoformat(),
-        "alternatives_considered": decision.alternatives_considered,
+        "alternatives": [
+            {
+                "index": i + 1,
+                "description": alt,
+                "chosen": alt == decision.chosen_option,
+            }
+            for i, alt in enumerate(decision.alternatives_considered)
+        ],
     }
     if show_reasoning:
         data["rationale"] = decision.rationale