feat(issues): S2 MCP tools — system CRUD + issue/system wiring

Second slice of Issues + Systems (spec #825).

New mcp/tools/systems.py: create_system, list_systems, get_system (records
split into issues/tasks/notes), update_system (incl. archive via status),
list_system_records (kind/open_only filters), delete_system. Registered in
register_all; read tools (get_system, list_systems, list_system_records) added
to the read-only-key allowlist (write tools default-deny).

create_task/update_task: kind now accepts 'issue'; new system_ids (set-semantics
associations) and arose_from_id (provenance, 0=unchanged/-1=clear) args.
create_note/update_note: new system_ids arg (notes associate with systems too).
services/notes.create_note: arose_from_id passthrough (update_note already
handles it via setattr).

Tests: MCP system tools + create_task issue-wiring (kind/provenance/systems),
service layer mocked.

Refs plan 825 (S2).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/scribe/mcp/server.py

@@ -185,6 +185,7 @@ _READ_ONLY_TOOLS = frozenset({
     "list_persons", "list_places", "list_projects", "list_rulebooks",
     "list_rules", "list_tags", "list_tasks", "list_topics", "list_trash",
     "list_always_on_rules", "search",
+    "get_system", "list_systems", "list_system_records",
 })