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/render_anthropic.py

@@ -23,6 +23,7 @@ from nisse_dev.canonical import (
     Compaction,
     DelegationCompleted,
     DelegationStarted,
+    Fact,
     GateOpened,
     PlanSubmitted,
     StoredEvent,
@@ -152,6 +153,8 @@ class AnthropicMessagesRenderer:
                         {"type": "text", "text": f"[delegation handback]\n{handback}"},
                     )
                 ]
+            case Fact(text=text):
+                return [("user", {"type": "text", "text": f"[orchestrator] {text}"})]
             case Compaction(summary=summary):
                 return [
                     (