Allow thread execution overrides for all providers

[ymichael]

Summary

• remove the stale Claude-only gate for sticky thread model and reasoning overrides
• validate every override against the thread provider's active model catalog, while retaining the requirement to start a new thread when changing providers
• add route coverage for Codex, Claude Code, Pi, and ACP and document how sticky overrides differ from per-turn follow-up overrides

Context

The original implementation explicitly gated Codex out for v1. That policy became stale: follow-up requests already send changed model and reasoning settings through the same next-turn/runtime reconfiguration paths, and the Codex, Pi, and ACP resume adapters all apply those settings today.

Deleting the now-always-supported capability avoids rejecting future and dynamically configured ACP providers through a second provider allowlist.

Testing

• pnpm exec turbo run test --filter=@bb/server -- test/public/public-threads.execution-override.test.ts test/services/threads/thread-execution-override.test.ts
• pnpm exec turbo run test --filter=@bb/agent-providers --filter=@bb/templates
• pnpm exec turbo run typecheck --filter=@bb/server --filter=@bb/agent-providers --filter=@bb/templates

AGENT GENERATED: by GPT-5

[bb-slop-cop]

🚨 SLOP COP 🚨 · review

I am the Slop Cop. I am reviewing this pull request for security, code quality, performance, architecture, and end-to-end behavior.

[bb-slop-cop]

🚨 slopcop/review — Keep a restore gate for ACP execution overrides

This now accepts model changes for every dynamic ACP provider. The runtime classifies ACP model changes as session changes and sends thread/resume.

When an agent lacks loadSession, the bridge creates session/new. The next turn then loses prior provider context.

Please retain a capability gate for these agents. Alternatively, add a live ACP model change that does not replace the session.

Please add a custom ACP test without loadSession. The current route matrix only checks built-in catalog persistence.

[bb-slop-cop]

🚨 SLOP COP 🚨 · review

Plain English summary

This change lets each provider change a thread model and reasoning level for later turns. The server limits each model choice to the current provider.

Review

I found one high-impact correctness problem.

• Custom ACP agents without loadSession can lose provider conversation history after a valid override.
• ACP treats the model change as a session change. The runtime sends thread/resume.
• The bridge creates a new session when the agent cannot restore the old session.
• Keep a capability gate for these agents, or add a live option change path.
• Add a test with a custom ACP agent that lacks loadSession.

The security review found no issue. The performance review found no hot-path or scale problem.

The capability removal reduces a duplicate provider list. A runtime support check would better match each active provider session.

Validation

• The focused server suite passed 15 tests.
• The provider suite passed 11 tests.
• The template suite passed 44 tests.
• Type checks passed for the server, provider, and template packages.
• All current GitHub CI checks passed.
• The exact pull request SHA ran in a local development server.
• A browser PATCH returned 200 for a Codex model and reasoning update.
• The next Codex turn used gpt-5.6-luna with high reasoning.