Add --no-estimate opt-out flag for plan use command and persist
EstimationSkipped reason when estimation is skipped or fails.

Add EstimationReport Pydantic domain model (identical to #649) with
spec-aligned multi-dimensional output: cost range (min/max USD),
expected steps, expected child plans, rollback risk (0.0-1.0),
estimated duration in minutes, confidence (0.0-1.0), rationale, and
optional historical basis.

Add EstimationSkipped domain model with reason, timestamp, and
optional actor_name fields for tracking when estimation is skipped
via --no-estimate or when the estimation actor fails.

Add estimation_produced to DecisionType enum and STRATEGIZE_TYPES.

Add Alembic migration m6_006 adding estimation_report_json and
estimation_skipped_json columns to v3_plans and updating the
ck_decisions_type CHECK constraint.

Wire --no-estimate through plan use CLI: when set, creates
EstimationSkipped with reason and clears the estimation actor.
Update _run_estimation() to persist EstimationSkipped on actor
failure instead of only logging.

Add docs/reference/estimation.md documenting the estimation feature,
EstimationReport schema, EstimationSkipped schema, and CLI examples.

Add benchmarks/estimation_actor_bench.py with ASV benchmarks for
EstimationReport and EstimationSkipped operations.

Add 13 Behave scenarios (estimation_skip.feature) and 6 Robot
integration tests (estimation_skip.robot) covering EstimationSkipped
model, EstimationReport model, plan field mutual exclusion, and
--no-estimate CLI behavior.

ISSUES CLOSED: #209

@@ -0,0 +27,4 @@
+    """Add estimation_report_json and estimation_skipped_json columns."""
+    with op.batch_alter_table("v3_plans") as batch_op:
+        batch_op.add_column(
+            sa.Column("estimation_report_json", sa.Text(), nullable=True)

@@ -118,0 +210,4 @@
+    )
+    timestamp: datetime = Field(
+        default_factory=datetime.now,
+        description="When estimation was skipped",

@@ -642,0 +646,4 @@
+    estimation_report: EstimationReport | None = Field(
+        default=None,
+        description="Structured estimation report from the estimation actor",
+    )