MUL-6629: feat(agents): add per-agent starter prompts for chat (#7261)

* feat(agents): add per-agent starter prompts

Co-authored-by: Agent Op <agentop718@gmail.com>

* fix(agents): default omitted starter prompts

Co-authored-by: Agent Op <agentop718@gmail.com>

* fix(migrations): renumber agent starter prompts

Co-authored-by: Agent Op <agentop718@gmail.com>

* fix(chat): make starter prompt prefills reliable

Co-authored-by: Agent Op <agentop718@gmail.com>

* fix(agents): preserve starter prompt writes

Keep dirty starter-prompt edits across agent refetches and fail closed when an older server cannot persist them. Clarify that copied starter prompts have no dedicated CLI override.

Co-authored-by: Agent Op <agentop718@gmail.com>

* fix(migrations): avoid starter prompt prefix collision

Main now uses migration prefixes through 403, so move the starter-prompts migration to the next available prefix before requesting another review.

Co-authored-by: Agent Op <agentop718@gmail.com>

* fix(agents): preserve edits after failed save

Ignore optimistic instruction snapshots while a save is pending so a rejected update can roll back the cache without erasing the user's submitted starter prompts. Cover the delayed optimistic-update and rollback sequence for both label and prompt fields.

Co-authored-by: Agent Op <agentop718@gmail.com>

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Co-authored-by: Agent Op <agentop718@gmail.com>