docs: record Phase C cutover; close $IsWindows shim bug (Phase C9)

- CLAUDE.md: Linux host no longer runs pwsh; PS 5.1 constraints scoped to
  Windows guests + Windows runner.
- RUNBOOK: add "Phase C — Python equivalents" table; §6-§13 benchmark records
  kept as historical provenance.
- TODO §3.7: close $IsWindows shim bug (resolved-by-replacement — bench run
  bypasses the shim).
- plans: mark C1-C8 done + docs C9; flag operational cutover/uninstall pending.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

CLAUDE.md

@@ -10,6 +10,8 @@ Self-hosted CI/CD system running on a Windows 11 host with VMware Workstation. E
 
 **Current state**: The Python orchestrator (`src/ci_orchestrator/`) is the production entry point. The PowerShell scripts in `scripts/` are 3-line shims that forward to the Python CLI. All new logic goes in Python.
 
+**Phase C (PowerShell-free Linux host)**: the manual burn-in / benchmark / smoke / host-validation / credential-setup tooling that used to run on the Linux host via `pwsh` is now native Python: `bench run`, `bench measure`, `smoke run`, `validate host`, `validate guest`, `creds set`. The Linux host no longer needs `pwsh` for any operation — it may be uninstalled (`setup-host-linux.sh` no longer installs it by default; opt in with `--with-pwsh`). PSScriptAnalyzer lint runs on a Windows runner on-demand, not on the Linux runner.
+
 ---
 
 ## Development Commands
@@ -115,6 +117,8 @@ The config file is auto-discovered at `$CI_ROOT/config.toml` (Linux default: `/v
 
 ## PowerShell 5.1 Constraints
 
+These constraints apply to the `.ps1` / `.psm1` scripts that still run **inside Windows guests** and on the optional Windows runner. The **Linux host itself no longer runs any `.ps1`** (Phase C) — do not add PowerShell to the Linux host orchestration path.
+
 All `.ps1` / `.psm1` scripts run on **Windows PowerShell 5.1** (not Core/7). Every script must start with:
 
 ```powershell