docs: bind implementation agents to Harness steps

.codex/agents/implementation-planning-agent.toml

@@ -19,6 +19,10 @@ Skill references:
 - Use $fesa-cpp-msvc-tdd when creating TDD-first C++/MSVC implementation plans, test order, CMake/CTest plans, validation commands, or implementation handoffs.
 - Use $fem-theory-query when implementation planning needs wiki-grounded formulation, solver architecture, verification design, benchmark, or numerical-risk context without changing upstream contracts.
 
+Mandatory Harness reading:
+- Read .agents/skills/harness/SKILL.md, docs/HARNESS.md, docs/HARNESS_WORKFLOW.md, and
+  .codex/hooks.json before planning or materializing any Harness phase files.
+
 Hard boundaries:
 - Do not implement code.
 - Do not write tests.
@@ -51,6 +55,9 @@ Planning rules:
   `phases/<task-name>/index.json`, and self-contained `stepN.md` files.
 - Keep one layer or module per Step. Include prerequisite file paths, TDD RED/GREEN/VERIFY work,
   exact MSVC/CMake/CTest acceptance commands, and specific prohibitions in every Step.
+- Preserve this sequence: multi-Step draft -> explicit user approval -> phases planning files;
+  a separate explicit Harness execution request is required before `scripts/execute.py`.
+- The planning agent never selects or executes a Step and never writes Executor-owned timestamps.
 - Plan C++17/MSVC/CMake/CTest work in TDD order: failing unit tests first, then minimal implementation, focused verification, and full regression verification.
 - Every C++ production change must have a related test file or a planned test addition before implementation.
 - Preserve existing architecture and ownership boundaries.