docs(rfd): Add RFD 053 for auto-refresh conversation titles

[JeanMertz]

RFD 053 proposes periodic background re-titling of conversations that have accumulated a configurable number of new turns since their title was last generated.

The core design introduces a turn_interval threshold (default 5) under conversation.title.generate.auto_refresh. When a jp query run starts, a background TitleRefreshTask scans all conversations, identifies stale candidates (those where turn_count >= baseline + turn_interval), and re-runs LLM title generation for up to batch_size of them — sorted by least recently activated to avoid churning active conversations.

Key design decisions:

• A new title_generated_at_turn field on Conversation tracks the turn count when the title was last auto-generated, serving as the staleness baseline.
• A retain_current field in the title generation schema lets the LLM signal that the existing title is still accurate, advancing the checkpoint without replacing the title.
• turn_context (default 10) caps how many recent turns are sent to the LLM, keeping costs predictable for long conversations.
• Manual conversation edit --title "..." writes a ConfigDelta with turn_interval = 0, opting that conversation out of auto-refresh.
• The full pipeline — scanning, loading, LLM calls — runs off the critical path and respects CancellationToken to avoid delaying CLI exit.

RFD 020 is also updated to reflect that get_conversation_mut (not just get_events_mut) requires a &ConversationLock, as the title refresh sync phase needs to mutate conversation metadata under the same locking invariant.