Simplify saved sessions and context compaction

[fazxes]

Summary

• Save native conversation history in one append-only JSONL file with separate metadata and tool-result artifacts, replacing the old cache, pointer, and generation writers.
• Convert older saved chats once on writable resume, preserving their summaries, retained messages, and available tool-result bytes.
• Use one checkpoint transaction for automatic and manual compaction: automatic compaction continues the turn; manual compaction stops.
• Summarize older context while retaining complete recent chat and tool exchanges, including their result bodies after restart.
• Target 10% summary and 5% recent context, allowing an oversized newest exchange to remain intact when it fits the model's usable window.
• Use the currently selected model and provider credentials for compaction.
• Allow model requests up to 120 seconds for response headers while preserving cancellation and existing request deadlines.
• Save completed in-flight work before the checkpoint without replaying its effects or duplicating conversation records.
• Avoid duplicating a completed turn when a queued follow-up starts before its finalization event is processed.
• Recover one provider context overflow even after an earlier compaction in the same turn.
• Keep the full transcript available while sending the summary and retained recent context to the model.
• Keep explicit recovery separate from fresh prompts, including queued prompts and persistent-child work.
• Keep interrupted chats discoverable after a torn final record or a checkpoint before the first turn finishes.
• Refresh the session list without rescanning it on every page operation.