refactor(examples): DRY the acp-agent configs via base-core.yml + acp-tail.yml

The snapshot replay config duplicated most of base.yml + the acp tail just to
swap llm-deepseek → llm-replay. Factor the shared pieces:

- examples/base-core.yml: the providerless provider/tool core (llm, sessions,
  system-prompt, tools, agents, invariants, bash-local, tool-bash). base.yml is
  now base-core + the llm-deepseek adapter; the snapshot replay config is
  base-core + llm-replay. The replay config no longer hand-copies the core.
- examples/acp-agent/acp-tail.yml: agent-loop (no pre-created agents) +
  persistence + the ACP bridge/system-prompt, shared by cordis.yml and the
  replay config so the three acp-agent configs can't drift. Its persistence root
  is `$DSH_SNAPSHOT_SESSIONS_ROOT ?? ./.sessions`.
- Deleted cordis.snapshot-record.yml: recording now reuses the normal cordis.yml
  (real adapter), with the harness redirecting the persistence root via env.
  start.ts maps DSH_SNAPSHOT=record → cordis.yml.

Verified: snapshot replay 8/8 keyless; record path works through cordis.yml;
ACP e2e no-key boot green through the doubly-nested include (cordis.yml →
base.yml → base-core.yml); coding-agent boots clean; all gates pass.

AGENTS.md

@@ -41,7 +41,9 @@ examples/    Runnable demos (not workspaces). echo-agent = mock model + echo
              (pnpm run demo:coding, needs DEEPSEEK_API_KEY).
              acp-agent = the coding agent exposed as an ACP server over
              JSON-RPC stdio (pnpm run demo:acp, needs DEEPSEEK_API_KEY).
-             base.yml = shared provider/tool core both real demos include.
+             base.yml = shared provider/tool core both real demos include
+             (= base-core.yml, the providerless core, + the llm-deepseek adapter;
+             base-core.yml is reused by the acp-agent snapshot-replay config).
 docs/        architecture.md — the design doc. module-graph.md — generated
              inter-package dependency graph (Mermaid; `pnpm run gen-module-graph`).
              rfc/ — design decisions and proposals, one kind of doc grouped by