FM: Subject switching — explicit command + agent-driven pivot #30

Phase 1a — explicit command. Literal match on "new subject" / "new topic" / "change subject":

1. Finalize + complete the current L3 session summary
2. Clear session's verbatim turns and L2s from payload
3. Return clean payload: persona + a few recent L4 project summaries

Phase 1b — agent-driven. Agent detects "on the subject of X" and re-queries Ragger with X; Ragger returns X's support payload. One extra round trip, but zero changes to assembly logic.

Phase 2 (future) — Ragger-intercepted. Detect subject-switch cues inside the assembly, pivot internally, no round trip. Only after defaults are tuned from real usage.

Note: implicit topic shifts are already handled by L3 merge-failure in Phase 2.