feat: retry the last user event

[Frozair]

Summary

• add a first-class API::retry(conversation_id) path instead of routing /retry through an empty follow-up message
• rebuild retry requests from the last saved raw user event in conversation history, skipping synthetic user messages that do not preserve raw content
• wire both retry entry points in forge_main to the new API method and add focused regression tests for retry event extraction

Why

The current interactive retry flow is reachable, but it retries by sending an empty follow-up into the conversation. This PR keeps /retry tied to the last actual user event, which is a closer semantic match for a real retry and avoids depending on a synthetic empty message.

Verification

• cargo test -p forge_domain test_last_retryable_event
• cargo test -p forge_main retry
• cargo check -p forge_api -p forge_main
• git diff --check

/claim #389

[Frozair]

Closing this out because the linked issue was already closed before submission, so I’m not planning to spend more review/follow-up time on this lane.