feat(agents-api): support public self-hosted initial input

[SaladDay]

Public self-hosted Session creation now accepts initial text in ordinary and streamed requests. It commits the existing reservation and returns the Environment connection target while offline; the Worker prepares and starts execution after connection. Initial expiry remains a failed Session without a fabricated Turn.

Both creation paths reuse current configuration, ownership, retry and event machinery. Saved-Agent recorded retries still recover before fresh admission; inline retries retain their existing validation. No schema, queue, native protocol or dependency changes.

Validation:

• Real PostgreSQL, actual API/Worker and fixed SDK 3.13.0/raw HTTP: concurrent creation, ordered messages, creation-event order, disconnect survival, saved-reference retries, reopen identity, controlled deadline failure, tenant boundaries and lease-loss admission. The new fixture is enrolled in CI.
• Built standalone service, registered daemon, pinned native executor and real MiniMax: ordinary initial, streamed initial and existing empty-create/later-input workflows, each with two remote command/file/history Turns and no retry side effects; existing none initial-input and service-restart regression.
• OpenAPI generation, focused race checks, standalone build, official suite and full make check.

This completes ENV-INITIAL-PUBLIC-001 within the current Codex text profile. Exact hosted timing/error/crash behavior and broader self-hosted capabilities remain unverified. Feishu task board.