chore: archive completed tasks, stale handoffs, and stray files

- heartbeat-n1-after-autonomous, n100-archivist-compression moved to
  tasks/archive/ — the first landed this session, the second was
  already built before it.
- Dated session handoffs (May 14c/15/16) moved to archive/handoffs/.
- Rescued a stray bug log to archive/bugs.md and removed the stray
  nested souveraine/ directory.
- Filed stray working files (an old chat.rs copy, a one-off script,
  terminal-paste garbage, a prev log) under docs/archive/strays/,
  which is gitignored — kept on disk, out of the repo.
- INDEX.md task list refreshed to drop archived references.

.gitignore

@@ -51,3 +51,7 @@ voice-recording-*.mp3
 # Web assets (if present)
 web/dist/
 souveraine.log
+souveraine.log.prev
+
+# Filed-away stray working files — kept on disk, out of the repo
+/docs/archive/strays/