Give each role its own view of the transcript (M1.2)

A model no longer reads another role's turns as its own (#78). The log
stays one record per work item; what a role is shown is projected from
it by Transcript.view() (views.py), which the planner, executor and
helper now build their context from:

- own events unchanged;
- operator messages labelled with their channel, e.g.
  [operator, Matrix] or [operator, comment on PR #165];
- orchestrator facts (a new `fact` event) as [orchestrator] statements;
- the executor sees a helper's handback as [helper report], not its turns;
- a helper sees the approved plan and the executor's milestone work as
  one-line narration;
- the planner sees no other role's work at all.

The orchestrator now writes facts at the points the models must know
about: work item opened, plan approved or rejected, milestone started,
finished or stopped, published (with PR number and URL) or not, review
comment answered, next milestone, task completed or abandoned.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01PPQ14L3LikyefCMFBqEbKE

nisse-dev/src/nisse_dev/executor.py

@@ -128,7 +128,7 @@ class LLMExecutor:
             f"Milestone {milestone} is stuck. The local executor tried and its "
             f"commands failed {failures} times in a row.\n\n"
             f"Most recent failure:\n{last_failure[:2000]}\n\n"
-            "The approved plan and the full transcript are in the conversation "
+            "The approved plan and the milestone so far are in the conversation "
             "above. Finish this sub-task, staying inside the approved plan."
         )
         result = self._delegate.run(
@@ -214,7 +214,7 @@ class LLMExecutor:
                 return ExecutorOutcome("stopped", f"before turn {turn + 1}")
             try:
                 produced = self._client.complete(
-                    transcript.read(after=after),
+                    transcript.view(after=after),
                     tools=tools,
                     model=model,
                     system=system,