feat(plan): register the plan session-projection unit

plan-mode contributes the RFC's complete double-event example: command/run
records named plan set the wanted target (off -> false, else true),
plan/mode commits and clears it, and view derives {active, pending}.
Pending is thereby a pure replay quantity recovered from the log alone.
The plan key merges into SessionProjectionMap from src/types.ts, served
through the new ./types and ./client outlets (session-title template);
compositions without the registry are unaffected.

packages/plan/plan-mode/src/client.ts

@@ -0,0 +1,10 @@
+/**
+ * Client-namespace projection of the plan domain: a pure re-export of the package's
+ * types outlet. Client code imports ONLY the client namespace (repo
+ * discipline), so `./client` projects the same single-source content
+ * `./types` serves to host consumers — zero duplication.
+ *
+ * @module @deepseek-ai/dsh-plan-mode/client
+ */
+
+export type * from './types.ts'