fix(cli): implement plan diff --correction to show real correction attempt diff

Implement spec-compliant correction diff output for `agents plan diff
--correction <CORRECTION_ATTEMPT_ID>`. Fixes the following issues from
the cycle-1 PR review:

- C1/M1: Replace direct `unit_of_work.correction_attempts` access with
  the proper `unit_of_work.transaction()` context manager, eliminating
  the AttributeError crash and the resource (session) leak.
- C2: Add `unit_of_work: UnitOfWork | None = None` constructor parameter
  to `PlanApplyService` and wire it in `_get_apply_service()` via
  `container.unit_of_work()`, removing the illegal `get_container()`
  call inside the method body (ADR-003 DI violation).
- C3: Replace metadata serialization stub with a three-section structured
  diff (Correction Diff summary, Comparison table, Patch Preview) as
  specified in §agents plan diff of the specification.
- C4/M2: Add `features/plan_correction_diff.feature` with 6 BDD
  scenarios covering all 4 output formats plus plan-not-found and
  correction-not-found error paths.
- C5: Update the three existing BDD scenarios that tested old stub
  behavior to mock `_get_apply_service()` and assert the new output.
- C6: Rename branch to `bugfix/m4-plan-diff-correction-stub` per
  CONTRIBUTING.md convention.
- C7: Amend commit message with body and ISSUES CLOSED footer.
- C8: Narrow `except Exception` to `except CorrectionAttemptNotFoundError`
  to avoid masking programming errors.
- M3: Add `robot/plan_correction_diff.robot` and
  `robot/helper_plan_correction_diff.py` integration test covering rich,
  plain, and JSON formats and the not-found error path.
- M4: Type `_build_correction_diff_dict` parameter as
  `CorrectionAttemptRecord` instead of `Any`.
- M5: Change `fmt: str` to `fmt: Literal["rich", "plain", "json", "yaml"]`
  on both `diff()` and `correction_diff()`, with a `cast()` call in the
  CLI layer where Typer supplies a plain `str`.
- M6: Add `ValueError` guards for empty `plan_id` and
  `correction_attempt_id` at the top of `correction_diff()`.
- M7: Add blank line between `diff()` and `correction_diff()` method
  definitions.
- M8: Update PR description to reflect actual implementation.
- m1: Remove unused `plan` variable in `correction_diff()`.
- m2: Reduce three blank lines to two between top-level definitions in
  `plan_apply_service.py`.
- n1: Remove trailing whitespace from blank line in `plan.py`.

Quality gates: lint (ruff), typecheck (pyright strict), unit_tests
(Behave 632 features / 0 failures) all pass.

ISSUES CLOSED: #9085

features/plan_cli_commands_r2.feature

@@ -189,11 +189,12 @@ Feature: Plan CLI commands branch coverage (round 2)
   # plan_diff – correction flag present
   # ===================================================================
 
-  Scenario: r2plan diff shows correction stub when correction flag set
+  Scenario: r2plan diff shows correction diff when correction flag set
     Given r2plan-a mocked lifecycle service
+    And r2plan-the apply service correction_diff returns formatted output for "CORR1"
     When r2plan-I invoke diff with plan "P1" and correction "CORR1"
     Then r2plan-the command should succeed
-    And r2plan-the output should contain "Correction Attempt"
+    And r2plan-the output should contain "Correction Diff"
     And r2plan-the output should contain "CORR1"
 
   # ===================================================================