Add M0-M7 economy organ plan specs (stomach)

The economy organ was the least-documented named component in the
architecture — one line in bus-topology.md, a Pony primitive, and a
smoke-test wire. This adds 8 plan specs following the 10-section template:

  M0 hub, M1 ingestion, M2 digestion core (small-model),
  M3 cost ledger, M4 budget governor, M5 context yield,
  M6 provenance chain (S1/S2 compliance), M7 outer-bus exchange.

Also updates the plans README index and integration DAG, and resolves
bus-topology.md's open item about the organ's placement.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

core/docs/bus-topology.md

@@ -84,4 +84,6 @@ blocking in a protected action), **tasks = workers** that call into the organs
   structure that isn't a lookup "store"?
 - **MoRAG / GoDAGRAG language** (Haskell vs Crystal vs other).
 - Each outer organ's hand-off shape to Ada.
-- The stomach/economy organ's exact placement + which small model runs it.
+- ~~The stomach/economy organ's exact placement + which small model runs it.~~
+  **→ placed:** M-series (M0–M7) in `docs/plans/`. Outer organ on Ichor; small model TBD (M2).
+  See M0 (hub), M6 (provenance chain through digestion, S1/S2 compliance).