fix(windows): detect broken ccproxy binary and fall back to pipx

[ClintonSarkar]

fix(windows): detect broken ccproxy binary and fall back to pipx

The pre-built Windows ccproxy.exe v0.2.10 ships without any auth provider plugins (see CaddyGlow/ccproxy-api#75). This makes clawde auth fail with No auth providers installed in this CCProxy release even on a fresh install.

This PR makes the clawde installer detect the broken binary and automatically fall back to a working install via pipx (Python 3.11+). If Python/pipx aren't available, the installer prints a clear next-step message instead of silently leaving the user with a non-functional clawde auth.

Changes

install.ps1

• Test-CCProxyHasProviders $bin\ccproxy.exe — runs ccproxy auth providers and returns $true only if at least one provider name appears in the output (returns $false on the "No OAuth providers found" / "No plugins found" cases, or on non-zero exit).
• Test-PipxAvailable — checks for py/python on PATH at >= 3.11, plus pipx on PATH.
• Install-CCProxyViaPipx — runs pipx install ccproxy-api[plugins-claude,plugins-codex], then drops a tiny ccproxy.cmd shim into the clawde bin dir that forwards to the pipx-installed script. The broken ccproxy.exe is removed first.
• Install-CCProxy — after a successful install, calls Test-CCProxyHasProviders and, if false, dispatches to the pipx fallback (or prints a manual-fix message).

cli/clawde.ps1

• Find-Binary now falls back to a .cmd shim with the same basename when the .exe is missing. This lets the pipx-installed ccproxy.cmd shim work transparently with the existing clawde start / clawde status / clawde update paths.
• Cmd-Start wraps Start-Process in cmd /c <shim> when the resolved binary is a .cmd, so PID/log redirection still works.

README.md

• A short note under Requirements explaining the Windows situation and pointing at the pipx workaround.

Why this approach

The real fix needs to land in CaddyGlow/homebrew-packages (the upstream CI that builds the Windows binary). That's a separate PR over there (CaddyGlow/homebrew-packages#1) plus a docs PR on CaddyGlow/ccproxy-api (#76).

In the meantime, this is the path that gets Windows users to a working clawde auth without them having to discover the issue themselves. The fallback only triggers when the binary is detected as broken, so users on a fixed release are unaffected.

Test plan

• On a Windows machine with a known-broken v0.2.10 ccproxy.exe, run .\install.ps1. It should auto-detect the missing plugins and switch to the pipx shim.
• After install, clawde auth should reach the OAuth browser step (no "No auth providers installed" error).
• On a Linux/macOS box, bash install.sh is unchanged.