test(coding-agent): add queue characterization coverage

packages/coding-agent/test/suite/README.md

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+# Coding agent suite tests
+
+Use `test/suite/` for the new harness-based test suite around `AgentSession` and `AgentSessionRuntimeHost`.
+
+Rules:
+- Use `test/suite/harness.ts`
+- Use the faux provider from `packages/ai/src/providers/faux.ts`
+- Do not use real provider APIs, real API keys, network calls, or paid tokens
+- Keep these tests CI-safe and deterministic
+- Do not use or extend the legacy `test/test-harness.ts` path unless a missing capability forces it
+
+Organization:
+- Put broad lifecycle and characterization tests directly under `test/suite/`
+- Put issue-specific regression tests under `test/suite/regressions/`
+- Name regression tests as `<issue-number>-<short-slug>.test.ts`
+- Example: `test/suite/regressions/2023-queued-slash-command-followup.test.ts`