Added BDD feature file and step implementations for cost reporting in CLI commands.
- Plan status now includes cost metadata in JSON output
- Session show includes estimated cost in JSON output
- Both commands display cost information in rich output format

ISSUES CLOSED: #5250

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+
+  Scenario: Plan status JSON output includes cost field
+    Given a plan exists with cost metadata
+    When I run "agents plan status <plan_id> --format json"

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+"""Step implementations for cost reporting in CLI output."""

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+
+from behave import given, then
+from behave.runner import Context
+

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+    """Create a test session with token usage."""
+    # This step is handled by test fixtures
+    # The session should have token_usage populated
+    context.session_has_tokens = True

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+
+# ---------------------------------------------------------------------------
+# JSON output assertions
+# ---------------------------------------------------------------------------

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+def step_estimated_cost_formatted(context: Context) -> None:
+    """Verify ``estimated_cost`` is a numeric value in JSON output."""
+    assert context.result.exit_code == 0, (
+        f"Expected exit code 0, got {context.result.exit_code}: "