Add default-off GPT-5.6 Ultra agent mode

[iam-brain]

Summary

• add the catalog-gated GPT-5.6 Ultra agent mode as a work in progress
• keep Ultra hidden behind runtime.ultra, defaulting to false, and map logical Ultra requests to the supported max wire effort
• use message-correlated session lineage and fail-closed catalog eligibility for root, child, and auxiliary agent policy
• retire the previous collaboration-profile/orchestrator prototype and safely clean up only plugin-managed legacy artifacts
• sync the OpenCode v1.17.18 plugin contract and upstream-watch coverage
• prepare the accumulated release notes for version 1.9.0

User and developer impact

Normal installs retain existing behavior because Ultra is disabled by default. Enabling runtime.ultra exposes Ultra only for eligible live-catalog models. The old managed orchestrator.md, collaboration headers, and prompt cache are removed; the core FetchOrchestrator retry and account-rotation engine is unchanged.

Safety and compatibility

• stale Ultra delegation overlays are stripped when eligibility cannot be proven or the feature is disabled
• ordinary instructions remain byte-for-byte unchanged when no Ultra overlay is present
• disabled accounts, session affinity, request metadata, and account rotation invariants remain covered
• user-authored orchestrator files are preserved during legacy cleanup

Validation

• npm run verify
• npm run check:upstream
• 127 test files passed; 875 tests passed, 1 skipped
• independent release review: no material findings
• npm pack --dry-run --json

Release

Planned as v1.9.0 after merge and green push CI on main, using the repository-native npm run release:minor flow.