fix(codex): discourage early exec yielding for one-shot commands

[cpacker]

Summary

• Adds a short Letta-specific guidance sentence to exec_command so ordinary one-shot commands omit yield_time_ms and wait for completion.
• Updates the unified exec description alignment test to document the intentional deviation from the upstream Codex wording.
• Fixes local backend OpenAI model resolution for release-dated registry handles like openai/gpt-5-mini-2025-08-07, which the Pi OpenAI catalog exposes as undated ids like gpt-5-mini.

Why

Upstream Codex exposes yield_time_ms tersely as a wait-before-yield knob. In Letta Code we observed GPT-5.5 applying yield_time_ms: 1000 to ordinary gh run view ... --json jobs --jq ... commands. Because those commands often take ~1.5s, each one returned Process running with session ID ..., then the next model turn issued empty write_stdin polls. In the three-command reproduction this produced three background sessions plus three follow-up polls for commands that would have completed inline with the default wait.

This intentionally strays from the source/reference tool description by adding model-facing guidance while preserving the Codex-compatible schema and runtime behavior.

The local CI failure was a separate model-catalog mismatch exposed by this PR run: gpt-5-mini-medium resolves through models.json to openai/gpt-5-mini-2025-08-07, but the local Pi OpenAI catalog exposes gpt-5-mini. Local backend model resolution now falls back from OpenAI release-dated ids to the undated catalog id when the exact id is absent.

Test plan

• bun test src/tools/codex-unified-exec-toolset.test.ts
• bun test src/backend/pi-model-factory.test.ts
• bun run check
• bun run src/test-utils/headless-scenario.ts --backend local --model gpt-5-mini-medium --output text --parallel on (local run skipped because this environment lacks OPENAI_API_KEY; CI will exercise it with secrets)
• Local model probe with Big Chungus the 3rd (agent-local-c47c57d5-72c5-4f23-baea-3fb1d441273e) on the exact three Caren screenshot commands:
  • Before this description guidance: local transcript showed three exec_command calls with yield_time_ms: 1000, three Process running with session ID ... results, and three empty write_stdin polls.
  • After the longer additive guidance and after this shorter guidance: local transcripts showed three exec_command calls with no yield_time_ms, no write_stdin polls, and inline completion in ~1.4-1.6s each.

👾 Generated with Letta Code