Add Fork conversation action to assistant messages

[Paul Lizer (paullizer)]

Summary

• add a personal-only Fork conversation action to persisted assistant messages
• copy authoritative history through the selected response with independent message, artifact, thread, and blob identities
• enforce ownership and stable-source checks with publish-last cleanup on failure
• add confirmation, duplicate-click prevention, immediate navigation, tests, documentation, release notes, and version 0.250.074

Validation

• 16 targeted tests passed; authenticated UI test collected and environment-skipped
• Python and JavaScript syntax checks passed
• XSS and broken-access-control committed-diff guardrails passed

Closes #1025