test: add e2e test for error handling when MCP servers are unavailable

## Problem

There are no e2e tests covering how the mirroring subsystem behaves when an MCP server is unreachable, times out, or returns errors.

## Proposed Solution

Add e2e tests in `tests-e2e/` that verify graceful error handling when:
- MCP server is unreachable (connection refused)
- MCP server times out
- MCP server returns malformed responses

## Files

- `tests-e2e/`
- `src/stuntdouble/mirroring/`

## Difficulty

Medium

Start in tests-e2e/ and inspect the related mirroring code under src/stuntdouble/mirroring/. Add end-to-end coverage for an unreachable MCP server, a timeout, and malformed responses, verifying graceful error handling in each case. Done means all three failure scenarios are exercised and the tests pass.

Written by the indexing model from the issue text.