feat: upgrade PR review to Opus 4.8 and issue answers to Sonnet 4.6 (engine v1.6) (#757)

* Bump Shadow engine to v1.4 for Opus 4.8 support

BEDROCK_MODEL_ID is now us.anthropic.claude-opus-4-8; v1.4 carries the
sampling-param fix that 4.8 requires (4.8 hard-fails on earlier pins). Reporter
stays Haiku 4.5.

* Bump engine to v1.6 + set Sonnet 4.6 for issue answers

Consolidates the model upgrades into one change:
- Engine pin v1.4 -> v1.6 (v1.6 includes the Opus 4.8 sampling-param fix and
  the issue-model wiring).
- .shadow.yml models.issue = Sonnet 4.6 for customer issue/followup answers,
  selected by an n=20 3-judge bench (14-5 over Haiku, half the hallucinations).
  PR reporter stays Haiku (JSON formatting only).
- Opus 4.8 for PR review remains applied post-merge via the BEDROCK_MODEL_ID
  secret (env > yaml).

* Drop trailing comments on reporter/issue model lines

Author: sudsali

.github/workflows/issue-bot.yml

@@ -40,11 +40,11 @@ jobs:
       (github.event_name == 'workflow_dispatch') ||
       (github.actor != 'github-actions[bot]' &&
        (github.event.issue.pull_request == null || github.event_name == 'pull_request_target'))
-    uses: sudsali/shadow/.github/workflows/shadow-review.yml@0ff3d759e3860fe5439682f51b6cd6352bf271c3
+    uses: sudsali/shadow/.github/workflows/shadow-review.yml@54ec94e0ca8c90d9b58ff95a2a06b175a115784e
     with:
       pr_number: ${{ inputs.issue_number }}
       dry_run: ${{ inputs.dry_run && 'true' || 'false' }}
-      shadow_ref: 0ff3d759e3860fe5439682f51b6cd6352bf271c3
+      shadow_ref: 54ec94e0ca8c90d9b58ff95a2a06b175a115784e
       aws_region: us-east-1
       prompt_sm_prefix: deequ-bot
     secrets:

.shadow.yml

@@ -17,3 +17,4 @@ models:
   investigator: us.anthropic.claude-opus-4-7
   critic: us.anthropic.claude-opus-4-7
   reporter: us.anthropic.claude-haiku-4-5-20251001-v1:0
+  issue: us.anthropic.claude-sonnet-4-6