[Fix] Fast automations ignore the configured session model

[roomote-roomote]

​Opened on behalf of @daniel-lxs. Follow up by mentioning @roomote-roomote, in the web UI, or in Slack.

What changed

• Persist custom automation model and reasoning selections when creating the Fast conversation, for both manual and scheduled runs.
• Reuse stored settings for initial and resumed Fast inference, while preserving explicit turn overrides and deployment-default reset semantics.
• Remove automation-specific model injection into delegated coding tasks. Children retain their independent explicit selection or deployment coding default.
• Add persistence, inference, trigger, and child-selection regression coverage, documentation, and a patch changeset.

Why this change was made

The automation override was only carried as a child-task default in the initial event. It never reached the Fast orchestrator, which used the deployment default instead. Storing the selection on the conversation makes it available on every turn without depending on the initial event payload.

Impact

New automation occurrences honor the configured Fast model throughout the conversation. Existing session settings are not overwritten, and historical occurrences are not backfilled. Schedules and provider routing are unchanged.

Validation passed: 357 focused tests, including real-database persistence and inference-boundary coverage; full pre-push lint/type/unused-code checks; and documentation validation/link checks. Removing the creation-setting handoff reproduced both manual and scheduled regression failures before restoring the fix. No live production automation was rerun, and no provider inference smoke test was performed. Browser proof is not applicable to model propagation.

[roomote-community]

No code issues found. See task

_{Reviewed d546e46}