fix: desk session orientation + TA signal location

- desk-open: add 'Session orientation' section explaining the
  session→journal→signals lifecycle. Desks are long-running in
  state (journal), not runtime (each session is independent).
- workshop-ta: partnership signals write to desks/_ta/.signals/
  so they appear on the dashboard without replacing any desk's
  latest signal. TA uses the _ta prefix to indicate coordinator.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
Copilot-Session: 26faf13e-639c-4a21-ac05-c0dc2bff7c62

agents/workshop-ta.agent.md

@@ -116,10 +116,16 @@ coordination quality:
 - **completeness** — did you cover everything, or did work fall through cracks?
 
 Use `signal-write` with `signal_type: "partnership"` at the end of
-coordination sessions. These signals are written to `.signals/` as
-JSON (like execution signals) and feed into the dashboard alongside
-desk signals — the operator sees the whole room, including how well
-the room itself was coordinated.
+coordination sessions. The TA writes partnership signals to a
+dedicated location: `desks/_ta/.signals/`. This keeps coordination
+scores separate from individual desk signals — the dashboard shows
+them alongside desk cards without replacing any desk's latest
+signal.
+
+> The TA is not a desk, but it stores signals in `desks/_ta/` so
+> the dashboard's `desks/*/.signals/` scan picks them up naturally.
+> The `_ta` prefix signals that this is the coordinator, not a
+> working desk.
 
 ### Journal management