fix review findings: stale coding-agent README, export-shape guards, overclaim wording

Codex review of PR #88 found three issues in the example-app extraction:

A1 — examples/coding-agent/README.md's plugin table still listed the OLD
direct-wired leaf entries (agent-loop, session-persistence, src/stdio-chat.ts —
the whole src/ dir is gone). Rewrite it to the four real leaf entries the
current cordis.yml loads (hmr, llm-deepseek, bash, stdio-agent), noting that
tool-bash/persistence/agent/loop now live inside the agent-core + stdio-agent
bundles.

A2 — the three new app/spine packages (agent-core, stdio-agent, acp-agent)
export NO `inject`, so a stray `export default apply` would let unwrapExports
collapse the module and silently DROP name/Config WITHOUT crashing — the
real-load-path smokes would stay green. agent-core is never Loader-unwrapped at
all. Add an explicit export-shape guard per package: assert no `default` export
and that the real Loader.unwrapExports leaves name/Config/apply intact. Verified
each fails when `export default apply` is added.

B — soften "structurally unreachable / cannot wire a stdout logger" overclaims
in the acp-agent/agent-core READMEs and the implemented RFC: a leaf CAN still add
a sibling logger entry; the accurate claim is the app omits one so the default
leaf has nothing to get wrong. Keep the safety directive (never add a stdout
logger to an ACP leaf).

packages/ui/acp-agent/README.md

@@ -16,7 +16,7 @@ stdout is the ACP JSON-RPC channel, so the cluster is defined as much by what it
 | ~~console logger~~ | **omitted** — it writes to stdout and would corrupt the protocol frames ([the stdout-purity footgun](../acp/README.md)) |
 | ~~`hmr`~~ | **omitted** — the editor owns the subprocess |
 
-Because there is no logger entry in the package, the footgun is **structurally unreachable from the leaf**: a leaf author cannot wire a stdout logger into the ACP config, because the leaf only picks backends, not the front door.
+Because the package wires no logger entry, an ACP leaf has **nothing to get wrong by default**: it only picks backends, so the common mistake — copying a console-logger entry from the stdio config — has no place here. (A leaf author technically *can* still add `@cordisjs/plugin-logger-console` as a sibling entry; the package can't forbid that. So the rule stands: never add a stdout logger to an ACP leaf — stdout is the JSON-RPC channel. Use a stderr exporter if you need logs.)
 
 ## Config