feat(inception): always-on exclusions reach every rule surface — list_always_on_rules(project_id), get_applicable_rules, rules_payload.excluded_always_on, session context, exclude/include tools (#2880, milestone 297 step 2)

A project that opted out of an always-on rulebook at inception must not see
it anywhere: list_always_on_rules(project_id=) and the subscription-derived
set skip it (an exclusion is total), get_applicable_rules / rules_payload
carry `excluded_always_on` as the seventh key so the departure is visible
wherever the rules are, and the SessionStart block built for a bound project
names it ("Excluded for this project by its inception decision …"). MCP:
list_always_on_rules takes project_id; exclude_always_on_rulebook /
include_always_on_rulebook mirror suppress/unsuppress (owner-only; the
rulebook must be always_on — subscribed rulebooks are left by unsubscribing).

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

src/scribe/services/plugin_context.py

@@ -1097,7 +1097,14 @@ async def build_session_context(
     at _MAX_CHARS with an explicit truncation note so the hook can pass it
     through verbatim.
     """
-    rules = await rulebooks_svc.list_always_on_rules(user_id)
+    # Inside a project, the always-on set is the project's: an inception
+    # exclusion (milestone 297) takes a rulebook out of this block, and is
+    # named below so the departure is visible rather than silent.
+    rules = await rulebooks_svc.list_always_on_rules(user_id, project_id=project_id)
+    excluded = (
+        await rulebooks_svc.excluded_always_on_rulebooks(user_id, project_id)
+        if project_id else []
+    )
     topic_map = await _topic_titles({r.topic_id for r in rules if r.topic_id})
 
     lines: list[str] = [
@@ -1119,6 +1126,12 @@ async def build_session_context(
             heading = topic_map.get(r.topic_id, "ungrouped") if r.topic_id else "ungrouped"
             lines.append(f"### {heading}")
         lines.append(f"- [{r.id}] {r.title}")
+    if excluded:
+        names = ", ".join(f"{e['title']} (#{e['id']})" for e in excluded)
+        lines += [
+            "",
+            f"Excluded for this project by its inception decision (not binding here): {names}.",
+        ]
 
     project_dict: dict | None = None
     if project_id: