Remove system strategy (legacy maps to developer). synthetic_tool still
forges the call but uses real stack text as the tool result, and now
yields ToolCall/Result events so the CLI flushes and does not glue nag
text to the next assistant stream.
Unset/empty url still maps to durable user-data chat.db.
Explicit :memory: is not rewritten; CLI warns when not quiet.
DatabaseConfig.url default is None; engine treats None as memory.
Add agent config compact_system_prompt, compact_user_prefix, and
compact_seed_text; wire through summarize and seed messages. Fix default
user prefix to include {transcript}; use safe seed substitution so braces
in summaries do not break formatting.