fix(iam): drop the decommissioned slack-bot deploy role from the stack (step 2/2)

Removes the SeahavenSlackBotDeployRole resource block. Step 1 recorded
DeletionPolicy/UpdateReplacePolicy Retain in the deployed template, so
CloudFormation stops managing the resource without issuing DeleteRole
against a role that no longer exists -- confirmed from the change set,
which reports PolicyAction: Retain on a single Remove entry.

seahaven-slack-bot was decommissioned in favour of sh-mcp and the role
was deleted directly in IAM on 2026-07-23. The stack is now consistent
with reality again, and stack updates no longer fail on it.

oidc-deploy-roles.yaml

@@ -1101,49 +1101,6 @@ Resources:
   # CDK deploy roles (4 repos)
   # ---------------------------------------------------------------------------
 
-  # DECOMMISSIONED — being removed from this stack in two steps.
-  #
-  # seahaven-slack-bot was retired (superseded by sh-mcp) and this role was
-  # deleted directly in IAM on 2026-07-23, leaving the stack holding a resource
-  # that no longer exists. That ghost broke EVERY subsequent stack update: the
-  # Outputs section resolved !GetAtt SeahavenSlackBotDeployRole.Arn as a live
-  # IAM read, which 404s. A change-set does not reveal this, because the
-  # resource itself is unchanged and Outputs are not previewed.
-  #
-  # Step 1 (this change): drop the Output so updates stop resolving the ghost,
-  # and record Retain so that step 2 cannot issue DeleteRole against a role
-  # that is not there.
-  # Step 2 (follow-up): delete the resource block itself. With Retain recorded,
-  # CloudFormation simply stops managing it — no IAM call is made.
-  SeahavenSlackBotDeployRole:
-    Type: AWS::IAM::Role
-    DeletionPolicy: Retain
-    UpdateReplacePolicy: Retain
-    Properties:
-      RoleName: githubdeploy-seahaven-slack-bot
-      AssumeRolePolicyDocument:
-        Version: "2012-10-17"
-        Statement:
-          - Effect: Allow
-            Principal:
-              Federated: !Sub arn:aws:iam::${AWS::AccountId}:oidc-provider/token.actions.githubusercontent.com
-            Action: sts:AssumeRoleWithWebIdentity
-            Condition:
-              StringEquals:
-                token.actions.githubusercontent.com:aud: sts.amazonaws.com
-              StringLike:
-                token.actions.githubusercontent.com:sub: !Sub repo:${GitHubOrg}/seahaven-slack-bot:ref:refs/heads/main
-      Policies:
-        - PolicyName: cdk-deploy
-          PolicyDocument:
-            Version: "2012-10-17"
-            Statement:
-              - Effect: Allow
-                Action:
-                  - sts:AssumeRole
-                Resource:
-                  - !Sub arn:aws:iam::${AWS::AccountId}:role/cdk-hnb659fds-*
-
   ExecAideDeployRole:
     Type: AWS::IAM::Role
     Properties: