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/canonical.py

@@ -73,6 +73,15 @@ class DelegationCompleted:
     exit_status: str
 
 
+@dataclass(frozen=True)
+class Fact:
+    """Something the orchestrator knows that the models must too (L6, L7):
+    a plan approved, a milestone finished, a PR opened. Written by the
+    orchestrator; every role sees it, as a statement rather than a turn."""
+
+    text: str
+
+
 @dataclass(frozen=True)
 class Compaction:
     summary: str
@@ -89,6 +98,7 @@ CanonicalEvent = (
     | GateOpened
     | DelegationStarted
     | DelegationCompleted
+    | Fact
     | Compaction
 )
 
@@ -101,6 +111,7 @@ EVENT_TYPES: dict[str, type] = {
     "gate_opened": GateOpened,
     "delegation_started": DelegationStarted,
     "delegation_completed": DelegationCompleted,
+    "fact": Fact,
     "compaction": Compaction,
 }