test/docs(shared-topics): dispatch test + journey + docs (D2)

Deterministic manager-core test (tests/shared_topics.rs) proves the
namespace boundary both ways: a shared publish fans out only to the
group's shared pubsub trigger (not a per-app or non-shared group
template), and a per-app publish never hits the shared trigger. CLI
journey (shared_topics) covers authoring + `pic triggers ls --group`
shared column + the two validation rejections (undeclared topic; shared
on an app), mirroring the shared_triggers norm. Docs: CLAUDE.md + design
doc §11.6 record D1 + D2 as shipped.

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

crates/picloud-cli/tests/cli.rs

@@ -43,6 +43,7 @@ mod routes;
 mod scripts;
 mod sealed;
 mod secrets;
+mod shared_topics;
 mod shared_triggers;
 mod staleness;
 mod stateful_templates;