fix(cli): clarify exception diagnostics (#2602)

Consolidate the CLI diagnostic plan, implementation, and test hardening into one reviewable change. The CLI now reports phase and target context for broad failure paths while preserving existing fail-fast behavior for real setup failures and warning-only behavior for optional best-effort work.

The workflow unit tests also avoid discovering real local agent CLIs, so developer machines with tools such as gemini installed do not hang pytest during metadata-only assertions.

Constraint: CLI setup failures must remain fail-fast, while optional preset and cleanup paths should continue with clear warnings.

Rejected: Replace broad handlers across the whole codebase in one pass | too broad for a targeted CLI diagnostic fix

Rejected: Add runtime timeouts to workflow agent dispatch | dispatch may legitimately be long-running and the observed hang was test isolation

Confidence: high

Scope-risk: moderate

Directive: Keep future best-effort CLI warnings tied to the failed phase and target so users can diagnose setup state.

Tested: uvx ruff check src/; uv run pytest tests/integrations/test_cli.py -v; uv run pytest tests/test_workflows.py::TestCommandStep::test_step_override_integration tests/test_workflows.py::TestPromptStep::test_execute_with_step_integration tests/test_workflows.py::TestPromptStep::test_execute_with_model -vv; uv run pytest

Not-tested: Real Nacos/PG/Redis-style external service failure injection; real interactive workflow dispatch against installed gemini CLI