fix(server-mode): close the gaps the review of workspace sharing found

- A Terminal tab a collaborator opens records them (dashboard_runs.started_by):
  its pane token acts as them — and only while still shared with them —
  never as the workspace owner (userFromPaneToken).
- POST /api/run always starts the lane's AI tool: a collaborator gets
  ECOLLABSTART whatever the body says (shell: true no longer slips past).
- A collaborator disabled since no longer locks the share list: kept on it
  (shown disabled) or taken off; only a new addition must be enabled.
- The workspace home tells a collaborator to open an existing session and
  keeps Start disabled instead of failing on every submit.

docs/plans/workspace-sharing/README.md

@@ -1,6 +1,12 @@
 # Workspace & session sharing — plan
 
 Status: **phase 1 built 2026-11-06** (workspace sharing; R1 decided as option **(a)** — accept and warn hard). Phases 2–4 not built.
+An independent review of phase 1 found and got fixed: a collaborator's Terminal
+tab acted as the owner through its pane token (now `dashboard_runs.started_by`),
+`POST /api/run {shell:true}` bypassed `ECOLLABSTART`, a disabled collaborator
+locked the share list, and the home's prompt box let a collaborator type into a
+Start that always failed. Accepted under (a): the owner's own panes' tokens are
+readable by a collaborator, and `ECOLLABSTART` is advisory under S3.
 
 Server mode only. In local mode there are no users, so there is nothing to share
 and every piece below is a no-op (`isServerMode()`).