[plan] [#917] add simplifier to the workflow

[ayazhankadessova]

It seems the Write, Edit, Bash, ExitPlanMode, and AskUserQuestion tools are not available to me in this session. I've completed a thorough analysis and have the full consensus plan ready. Let me try to present my findings as my final output since I cannot use the expected plan mode tools.

The complete consensus plan has been detailed above in my response. Here's the summary:

Task: Wire simp_kernel() into the impl FSM as an opt-in stage between review and pr.

All three agents converge on the same design with only minor LOC estimate differences (~240-295 LOC, consensus: ~280 LOC). There are zero conflicts between proposals.

7 steps, Docs → Tests → Implementation order:

1. Docs (~80 LOC) — 7 .md files
2. Tests (~80 LOC) — Update test_impl_fsm.py, add TestSimpStageKernel
3. state.py (~15 LOC) — Constants + Literal/tuple updates
4. checkpoint.py (~3 LOC) — Literal type update
5. transition.py (~20 LOC) — Reroute + new transitions + required_pairs
6. kernels.py (~50 LOC) — simp_stage_kernel() + KERNELS registration
7. impl.py + __main__.py (~30 LOC) — Params + CLI flags

Key pattern to reuse: review_stage_kernel passthrough at kernels.py:142-143 is the exact template.

Verification: python -m pytest python/tests/test_impl_fsm.py -v and python -m agentize.workflow.impl --help

Dude, carefully read my response to determine what to do next.
Plan based on commit 68501f3