fix(install): resolve PowerShell host instead of bare powershell for uv install

[xxxigm]

Summary

A Windows user reported the installer getting stuck; running
irm https://hermes-agent.nousresearch.com/install.ps1 | iex failed at the uv
step with:

[X] Failed to install uv: The term 'powershell' is not recognized as the name
    of a cmdlet, function, script file, or operable program.
[X] Installation failed: uv installation failed

Root cause. Install-Uv spawned the astral uv installer with a hardcoded
bare powershell:

powershell -ExecutionPolicy ByPass -c "irm https://astral.sh/uv/install.ps1 | iex"

The bare name powershell resolves only to Windows PowerShell, and only
when its System32\WindowsPowerShell\v1.0 directory is on PATH. When
install.ps1 is run under PowerShell 7+ (pwsh) — or any session where
powershell isn't on PATH — the spawn dies with "The term 'powershell' is
not recognized", so uv never installs and the installer appears stuck. The
script already documents that it can be invoked from "any x64 PowerShell host"
(see the Get-WindowsArch note), so assuming powershell is on PATH is the
inconsistency.

Fix

Add Get-PowerShellHostExe, which resolves the host executable in priority
order:

1. the absolute path of the host we're already running in
   ((Get-Process -Id $PID).Path) — PATH-independent, and matches the
   host the user actually launched (powershell.exe or pwsh.exe);
2. fall back to powershell / pwsh via Get-Command;
3. last-ditch bare powershell so the spawn surfaces its own error.

Install-Uv now invokes the resolved exe (& $psHostExe -ExecutionPolicy ByPass -c "irm https://astral.sh/uv/install.ps1 | iex"). The host filter only
trusts a current host whose leaf name is powershell.exe/pwsh.exe, so
powershell_ise.exe and other embedded hosts (which can't take
-ExecutionPolicy/-Command) fall through to Get-Command.

This is the only external powershell spawn in the script; the cross-process
stage driver lives in Hermes-Setup.exe, not here.

Test plan

• tests/test_install_ps1_uv_powershell_host.py (new) — source-level guards
  (the script only runs on Windows, so there's no Linux CI runner to execute
  it): the bare-powershell astral literal is gone, the installer is invoked
  via & $-resolved host variable, and the resolver is PATH-independent
  (Get-Process -Id $PID) and pwsh-aware.
• scripts/run_tests.sh tests/test_install_ps1_uv_powershell_host.py — 3 passed.