feat: expose Fusion custom tools to ACP agents via loopback MCP bridge (#3476)
## Summary
- ACP runtimes can expose Fusion custom tools (`fn_*`) to external
agents such as Hermes ACP and Prime.
- When the engine passes `customTools`, `AcpRuntimeAdapter` starts a
per-session loopback tool bridge and registers it as a stdio MCP server
in `session/new.mcpServers`.
- The bridge uses a per-session bearer token, binds only to loopback,
preserves the MCP request ID as `toolCallId`, propagates tool `isError`
results, and exposes only runnable non-built-in tools.
- The MCP shim supports `initialize`, `ping`, `tools/list`, and
`tools/call`, reporting transport, authentication, malformed-request,
and unknown-tool failures correctly.
- Bridge startup failures degrade gracefully: the ACP session remains
usable without custom tools and exposes a fixed `fusionToolBridgeError`
reason code for engine auditing.
- Startup, request, and teardown paths clean temporary schemas and
listeners; disposal aborts cooperative tools, bounds non-cooperative
drains, prevents post-dispose execution, and preserves the existing
synchronous session contract through an awaitable `disposePromise`.
- The packaged CLI stages `mcp-schema-server.cjs` beside the ACP runtime
distribution.
## Verification
Validated at source commit `2fa6fd2e26923ac6241d1b9379b3819f1832c079`:
- `pnpm --filter @fusion-plugin-examples/acp-runtime test` — 21 files /
251 tests passed
- `pnpm --filter @fusion-plugin-examples/acp-runtime typecheck` — passed
- Plugin build with `.cjs` asset copy — passed
- `pnpm check:changesets` — passed
- `git diff --check` — passed
- MCP shim smoke test — passed
- Greptile Review — passed
- CodeRabbit — passed
- Devin review — 0 bugs, 6 analyses, `errored_tasks=[]`, quality `4/10`
## Scope
ACP runtime plugin only. The Hermes CLI runtime remains untouched;
Hermes ACP is enabled by selecting the generic ACP runtime with
`acpBinaryPath: hermes` and `acpArgs: ["acp"]`.
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Co-authored-by: gsxdsm <gsxdsm@users.noreply.github.com>
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