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/SERVER-MODE.md

@@ -257,9 +257,17 @@ decides the account. They can read everything in the owner's home (the owner's
 other workspaces, SSH keys, AI logins, the pane tokens of the owner's panes)
 and their commits carry the owner's git identity; only AgentHub's `audit_log`
 records who did what. The Share dialog says so. Share only with people you
-would hand your keyboard to.
+would hand your keyboard to. Two consequences follow and are accepted, not
+bugs: a collaborator can read the pane token of a pane the OWNER started (and
+so act as the owner through the CLI from it), and `ECOLLABSTART` is a
+dashboard rule, not a barrier — in a terminal they can run `claude` as the
+owner's account. A Terminal tab the collaborator opens records them as its
+starter (`dashboard_runs.started_by`), so its own pane token acts as THEM, and
+only while the workspace is still shared with them (`userFromPaneToken`).
 
-Every share and unshare is audited (`share workspace <id>` /
+A user already on the list who was disabled since stays listed (marked
+disabled, granting nothing) and can be taken off; only a NEW addition must be
+enabled. Every share and unshare is audited (`share workspace <id>` /
 `unshare workspace <id>`). Taking it back sends the users taken off
 `lane_access` (their list re-reads) and closes their open terminals on it at
 the next attach re-check. Deleting the workspace, or the user, drops the rows;