[F090] drop auditor from write_roles on main-pm-board / board-private

The auditor is a silent, read-only observer on every channel, but the
channel catalog (roboco/foundation/policy/communications.py) listed it
in write_roles for main-pm-board and board-private 'for parity' with the
legacy CHANNEL_ACCESS table, while the actual silent-observer rule was
enforced only at the say/dm guard (content_actions._NO_COMMS_ROLES) and
PermissionService.can_write_channel's auditor short-circuit.

That left the catalog-only enforcement path — the HTTP messaging route
(messages.py send_message -> validate_channel_access) — authorizing an
auditor write that both the say/dm guard and PermissionService would
have blocked. A reader of the catalog also believed the auditor could
post to those channels, which is false.

Fix: remove Role.AUDITOR from write_roles on both channels (main-pm
+ board remain writers; ceo remains a writer on board-private). The
auditor stays in read_roles, so its silent read is unchanged. silent_roles
is left empty (matches the announcements precedent: auditor reads via
read_roles, not the silent bucket) — the DB seed and silent_observers
field are untouched.

Logical-regression check: the auditor's read access on both channels
is byte-for-byte preserved (still in read_roles, so validate_channel_access
read returns True via the direct list); the legitimate writers (main-pm,
product-owner, head-marketing, ceo) are untouched; CHANNEL_ACCESS is
derived from the spec so the foundation/seed drift tests self-adjust;
PermissionService.can_write_channel already short-circuited auditor to
False everywhere, so no behavior change there; AUDITOR_SILENT_ACCESS is
unchanged (auditor not added to silent_roles -> no DB silent_observers
change -> no group-access behavior change); the say/dm _NO_COMMS_ROLES
guard is unchanged. Tests: 3 new in test_channel_access.py — auditor
write on main-pm-board/board-private now raises ChannelAccessDeniedError
(RED before: returned True), auditor read still True, main-pm/ceo still
write.

roboco/foundation/policy/communications.py

@@ -219,9 +219,14 @@ CHANNELS: dict[str, ChannelSpec] = {
         silent_roles=_AUDITOR_ONLY,
     ),
     # -- Management channels --------------------------------------------------
-    # Legacy CHANNEL_ACCESS lists auditor as both read AND write here. We
-    # preserve that behaviour for parity; the runtime guard that downgrades
-    # auditor to silent lives in services.
+    # The auditor is a silent, read-only observer on every channel — its
+    # read membership is declared in read_roles, but it must NOT appear in
+    # write_roles. Previously the catalog listed the auditor as a writer
+    # here "for parity" with the legacy CHANNEL_ACCESS table, while the
+    # actual silent-observer rule was enforced only at the say/dm guard.
+    # That left the catalog-only enforcement path (the HTTP messaging route
+    # -> validate_channel_access) authorizing an auditor write that the
+    # guard would have blocked. The catalog now matches the enforced rule.
     "main-pm-board": ChannelSpec(
         slug="main-pm-board",
         description="Main PM and Board communication",
@@ -229,9 +234,7 @@ CHANNELS: dict[str, ChannelSpec] = {
         read_roles=frozenset(
             {Role.MAIN_PM, Role.PRODUCT_OWNER, Role.HEAD_MARKETING, Role.AUDITOR}
         ),
-        write_roles=frozenset(
-            {Role.MAIN_PM, Role.PRODUCT_OWNER, Role.HEAD_MARKETING, Role.AUDITOR}
-        ),
+        write_roles=frozenset({Role.MAIN_PM, Role.PRODUCT_OWNER, Role.HEAD_MARKETING}),
         silent_roles=frozenset(),
     ),
     "board-private": ChannelSpec(
@@ -247,9 +250,7 @@ CHANNELS: dict[str, ChannelSpec] = {
                 Role.MAIN_PM,
             }
         ),
-        write_roles=frozenset(
-            {Role.PRODUCT_OWNER, Role.HEAD_MARKETING, Role.AUDITOR, Role.CEO}
-        ),
+        write_roles=frozenset({Role.PRODUCT_OWNER, Role.HEAD_MARKETING, Role.CEO}),
         silent_roles=frozenset(),
     ),
     # -- Special / broadcast channels -----------------------------------------

tests/unit/enforcement/test_channel_access.py

@@ -75,3 +75,35 @@ def test_validate_channel_access_silent_observer_can_read() -> None:
     """Line 80: silent observers can read."""
     # backend-cell has 'auditor' as silent observer.
     assert validate_channel_access("auditor", "backend-cell", "read") is True
+
+
+# ---------------------------------------------------------------------------
+# Auditor is a silent, read-only observer — the catalog must not grant it
+# write_roles on any channel. main-pm-board / board-private used to list the
+# auditor in write_roles (legacy parity), which let the catalog-only
+# enforcement path (the HTTP messaging route -> validate_channel_access)
+# authorize an auditor write that the say/dm guard would have blocked.
+# ---------------------------------------------------------------------------
+
+
+@pytest.mark.parametrize("channel", ["main-pm-board", "board-private"])
+def test_validate_channel_access_auditor_cannot_write_management_channels(
+    channel: str,
+) -> None:
+    """Auditor must not be in write_roles for any channel — silent observer."""
+    with pytest.raises(ChannelAccessDeniedError):
+        validate_channel_access("auditor", channel, "write")
+
+
+@pytest.mark.parametrize("channel", ["main-pm-board", "board-private"])
+def test_validate_channel_access_auditor_can_still_read_management_channels(
+    channel: str,
+) -> None:
+    """Removing write access must not regress the auditor's silent read."""
+    assert validate_channel_access("auditor", channel, "read") is True
+
+
+def test_validate_channel_access_main_pm_still_writes_main_pm_board() -> None:
+    """The legitimate writers (main-pm / board / ceo) are untouched."""
+    assert validate_channel_access("main-pm", "main-pm-board", "write") is True
+    assert validate_channel_access("ceo", "board-private", "write") is True