Purge legacy pre-pivot scaffold (decided: extractor/merger-only architecture)

Delete src/extract/ (pi-session normalize/extract/merge stubs), src/ledger.ts,
src/schema.ts, src/derive.ts, src/queries/, src/cli.ts (npm run ingest/graph/
what-next), data/legacy-graph/, data/agent/ (pi agentDir vestige), and their
config.ts/package.json references.

The retroactive-bootstrap goal is dropped: the live chat store (AI SDK
UIMessages) is the only session format now, and the old pi JSONLs in
data/sessions/ stay on disk, inert. contracts.ts is the single schema.

package.json

@@ -8,9 +8,6 @@
 		"dev": "next dev",
 		"build": "next build",
 		"start": "next start",
-		"ingest": "tsx src/cli.ts ingest",
-		"graph": "tsx src/cli.ts graph",
-		"what-next": "tsx src/cli.ts what-next",
 		"test": "NODE_ENV=test tsx --test tests/*.test.ts tests/*.test.tsx",
 		"typecheck": "tsc --noEmit"
 	},