feat: bump platform default models to Claude Opus 5.5 / Haiku 5.5 #953

Summary

Move the platform default models to the latest Anthropic models available on Bedrock:

Both proposed models have ACTIVE global. and us. cross-region inference profiles (verified via aws bedrock get-inference-profile in us-east-1 on 2026-10-16). anthropic.claude-sonnet-5-5 and anthropic.claude-fable-5-1 are also live.

Follows the pattern established by #741 (Opus 5 default) and its children #744–#747. This is a fresh bump, not a reopening of that work.

Scope

• cdk/src/handlers/shared/bedrock-model-constants.ts
  • Add anthropic.claude-opus-5-5, anthropic.claude-sonnet-5-5, anthropic.claude-haiku-5-5 to DEFAULT_BEDROCK_MODEL_IDS (additive — keep opus-5, sonnet-4-6, opus-4-8, haiku-4-5 so repos pinned to them do not fail at turn 0).
  • Switch PLATFORM_DEFAULT_MODEL_ID and DEFAULT_HAIKU_MODEL_ID to the new values.
• agent/src/config.py (ANTHROPIC_MODEL, ANTHROPIC_DEFAULT_HAIKU_MODEL fallbacks) and agent/src/models.py (anthropic_model, haiku_model defaults): mirror the new literals.
• cli/src/repo-display.ts: mirror the display default.
• Tests under cdk/test/, agent/tests/, cli/test/ that assert the defaults or the grant list.
• docs/guides/ model-configuration prose; regenerate the Starlight mirror with mise //docs:sync.
• bgagent platform doctor must pass against the new grant set.

Acceptance criteria

• A task with no per-repo model pin runs on global.anthropic.claude-opus-5-5 with global.anthropic.claude-haiku-5-5 as the auxiliary model.
• Repos pinned to any previously allowlisted model continue to run unchanged.
• The IAM grant covers every model in DEFAULT_BEDROCK_MODEL_IDS in the deployed geography; bgagent platform doctor reports no missing inference profiles.
• mise run build and mise run security are green.
• Docs updated and mirror regenerated.

Open question

Should fable-5-1 be added to the grant allowlist (not as a default) in the same change? Default proposal: no, keep the allowlist to the Opus/Sonnet/Haiku 5.5 family; file a follow-up if a repo needs it.

Deployment note

Account-level Bedrock model access for the 5.5 models is a prerequisite in the deployment account; IAM grantInvoke alone is not sufficient.