/vibe (a full director/worker orchestration mode) had no user-facing
documentation, and /fresh appeared only as a matrix row in the
session-operations doc whose title already promised a "fresh" section.
Neither was mentioned in the README.
- Add docs/vibe-mode.md: enable/disable, fast/good worker tiers, the five
vibe_* control tools, and the director workflow.
- Add a "## Fresh" section to the session-operations doc describing the
provider-stream/session-state reset and its contrast with /new and /drop.
- Add a "Session controls" subsection to the README linking both docs.
Fixes#6440
can1357