feat: adapter contract, commerce proof-of-fit, and reusability docs

Proves the engine did not stay video-specific during extraction.

- application adapter contract in core (createApplicationAdapter,
  evaluateObject, collectTargets): an object names the targets that govern it
  and the strictest verdict reaching any of them wins, so denying a seller
  takes their whole catalogue down with no cascade logic in the application
- examples/commerce: a marketplace on the public API covering all ten
  proof-of-fit behaviors the plan requires, including that no core change was
  needed. One product, one snapshot, four surfaces, four different verdicts.
- README, integration guide, per-example README, CHANGELOG, four ADRs, and
  GPL-3.0-or-later LICENSE (the license was listed in Phase 1 but missing)
- checkpoints rewritten to reflect actual state and to say plainly which
  migration-plan phases belong to the consumer repos rather than here

Fixes a real gap found by the proof-of-fit: composeDecisions kept only the
first decision's evidence, so an object governed by several targets lost the
evidence that drove its verdict. Evidence now merges — counts summed,
contributor lists unioned, flags or-ed.

556 tests; lint, typecheck, and per-package type build clean.

examples/commerce/package.json

@@ -0,0 +1,15 @@
+{
+  "name": "@nostr-governance/example-commerce",
+  "version": "0.1.0",
+  "type": "module",
+  "private": true,
+  "description": "Proof-of-fit: the governance engine driving a marketplace, using only the public API.",
+  "main": "src/marketplace.js",
+  "scripts": {
+    "test": "vitest run"
+  },
+  "dependencies": {
+    "@nostr-governance/core": "*",
+    "@nostr-governance/runtime": "*"
+  }
+}