kb: retire pi-installation association (OMP is the primary harness)

Purge live-state claims that ~/.pi is our harness; keep general
pi documentation in omp-docs/ and dated historical records.

- README.md: single config source; Legacy Notes reworded
- AGENTS.md: drop 'like ~/.pi did' framing
- APPEND_SYSTEM.md: okf-spec -> omp-docs/references/okf-spec.md
- dotfiles/zsh/.zshrc: systemprompt alias -> APPEND_SYSTEM.md
- managed skill llm-fleet-model-swap + model_config_sync skill:
  single-source models.yml; validation script YAML-only
- memories (MEMORY.md, memory_summary.md, learned.md): single source
- omp-docs: .okf root -> ~/.omp/omp-docs; de-live pi-era claims in
  pi/index.md, pi-web-access.md, engineering/agents-files.md

agent/APPEND_SYSTEM.md

@@ -57,7 +57,7 @@ Suggest these tools or languages as possible solutions to problems if they are a
 The knowledge base at `docs/` is the source of truth for your project. Your training is out of date. Default to the documentation and the code before you assume anything. Maintain a work log of actions taken and their result. Refer to the work log to remember the progress of your tasks.
 
 1. Read the relevant `index.md` before you touch a topic.
-2. Follow `~/.pi/agent/docs/references/okf-spec.md` for every doc you create or edit.
+2. Follow `omp-docs/references/okf-spec.md` for every doc you create or edit.
 3. Read the target file completely before you edit it.
 4. Always update the documentation when you've completed a task.
 5. Compact the work log when necessary.