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

skills/desk-open/SKILL.md

@@ -45,6 +45,23 @@ desks/<desk-name>/
 4. **Announce it.** Tell the operator what was created and what
    the desk's focus is.
 
+## Session orientation
+
+This skill initializes storage — it does not launch a session.
+A desk becomes active when a Copilot session references its
+directory. The session workflow:
+
+1. The operator (or TA) starts a session and says "sit at the
+   `<desk-name>` desk"
+2. The session reads `desks/<desk-name>/journal.md` to load priors
+3. Work happens — the session uses `signal-write` to emit signals
+   and `desk-journal` to persist state at the end
+4. The next session repeats from step 2
+
+The desk identity comes from which journal is read, not from a
+persistent process. Desks are long-running in *state* (the journal
+carries forward), not in *runtime* (each session is independent).
+
 ## Journal format
 
 ```markdown