Replace the single flat 'Decision Details' table in explain_decision_cmd()
with six structured Rich panels matching the spec exactly:

- Decision panel: ID, Type, Question, Chosen, Confidence, Plan, Sequence, Created
- Alternatives Considered panel: numbered list with '(chosen)' marker
- Impact panel: Downstream Decisions, Downstream Child Plans, Artifacts Produced,
  Correction Impact (derived from downstream_decision_ids count)
- Context Snapshot panel (--show-context): resource paths + hot context hash
- Rationale panel (--show-reasoning): rationale text
- Correction panel: 'agents plan correct <ID> --mode revert --guidance "..."' hint

Also adds the missing success message '✓ OK Decision explained'.

Updates _build_explain_dict() to:
- Accept total_decisions parameter for 'X of Y' sequence format
- Return structured alternatives list [{index, description, chosen}] instead
  of flat string list
- Include impact dict with downstream_decisions, downstream_child_plans,
  artifacts_produced, correction_impact
- Include correction_hint field

Updates feature files and step definitions to test the new output structure.

ISSUES CLOSED: #2815

@@ -3471,7 +3526,9 @@ def _build_explain_dict(
         "is_correction": decision.is_correction,
         "superseded": decision.is_superseded,
         "created_at": decision.created_at.isoformat(),

@@ -3545,3 +3608,4 @@
 
     if fmt in (OutputFormat.JSON, OutputFormat.YAML, OutputFormat.TABLE):
         console.print(format_output(data, fmt))
+    elif fmt == OutputFormat.PLAIN.value or fmt == "plain":

@@ -3548,0 +3625,4 @@
+            "Alternatives Considered",
+        ]
+        plain_alts = data.get("alternatives", [])
+        assert isinstance(plain_alts, list)

@@ -3540,3 +3601,4 @@
+
     data = _build_explain_dict(
         decision,
         show_context=show_context,

@@ -3540,0 +3597,4 @@
+    # Fetch total decision count for "X of Y" sequence format.
+    total_decisions: int | None = None
+    with suppress(Exception):
+        total_decisions = svc.count_decisions(decision.plan_id)

@@ -3548,0 +3618,4 @@
+            f"  Question: {data['question']}",
+            f"  Chosen: {data['chosen']}",
+            f"  Confidence: {data['confidence']}",
+            f"  Plan: {data['plan_id']}",

@@ -3540,0 +3597,4 @@
+    # Fetch total decision count for "X of Y" sequence format.
+    total_decisions: int | None = None
+    with suppress(Exception):
+        total_decisions = svc.count_decisions(decision.plan_id)

@@ -3563,0 +3739,4 @@
+            console.print(
+                Panel(
+                    rationale or "(no rationale recorded)",
+                    title="Rationale",

@@ -3542,3 +3604,4 @@
         show_context=show_context,
         show_reasoning=show_reasoning,
+        total_decisions=total_decisions,
     )

@@ -3537,29 +3594,167 @@ def explain_decision_cmd(
         )
         raise typer.Exit(1)
 
+    # Fetch total decision count for "X of Y" sequence format.