feat(actor): implement estimation actor type (#962)

## Summary

Implement the estimation actor as a functional actor type. The estimation actor provides cost, time, and resource estimates for plan operations, running after Strategize completes (before Execute). Estimation is informational only and optional — plans work without an estimation actor configured.

### Changes

**New files:**
- `src/cleveragents/domain/models/core/estimation.py` — `EstimationResult` frozen Pydantic model with fields for cost (USD), tokens, steps, child plans, time, risk level/factors, summary
- `features/estimation_actor.feature` — 12 Behave test scenarios (model validation, serialization, stub actor, plan integration, optional behavior)
- `features/steps/estimation_actor_steps.py` — Step definitions
- `robot/estimation_actor.robot` — 6 Robot integration tests
- `robot/helper_estimation_actor.py` — Robot test helper

**Modified files:**
- `domain/models/acms/tiers.py` — Added `ESTIMATOR` to `ActorRole` enum
- `domain/models/core/plan.py` — Added `estimation_result: EstimationResult | None` field, estimation data in `as_cli_dict()`
- `application/services/plan_executor.py` — Added `EstimationStubActor` class
- `application/services/plan_lifecycle_service.py` — Added `_run_estimation()` method, invoked in `execute_plan()`
- `cli/commands/plan.py` — Display estimation results in plan status
- `vulture_whitelist.py` — Added 12 new public symbols

### Quality Gates

| Session | Result |
|---|---|
| `nox -s lint` | PASS |
| `nox -s typecheck` | PASS (0 errors) |
| `nox -s unit_tests` | PASS (10,818 scenarios) |
| `nox -s integration_tests` | PASS (1,512 tests) |
| `nox -s coverage_report` | 98% (>= 97%) |

Closes #890

Reviewed-on: #962
Co-authored-by: Brent Edwards <brent.edwards@cleverthis.com>
Co-committed-by: Brent Edwards <brent.edwards@cleverthis.com>

features/context_tiers.feature

@@ -10,8 +10,8 @@ Feature: ACMS Context Tiers
 
   # ---- ActorRole enum ----
 
-  Scenario: ActorRole has three values
-    Then ActorRole should have values "strategist", "executor", "reviewer"
+  Scenario: ActorRole has four values
+    Then ActorRole should have values "strategist", "executor", "reviewer", "estimator"
 
   # ---- TieredFragment ----