fix(install): capture uv stdout+stderr via Start-Process so install failures are diagnosable

[jrob5756]

Problem

Reported by a user hitting an install failure on Windows: `install.ps1` reports

```
Install failed after 3 attempts.
── uv tool install output ──
(no output captured)
```

… leaving the user with no way to diagnose the failure.

Root cause

`install.ps1` sets `$ErrorActionPreference = 'Stop'` at the top, then runs:

```powershell
$output = $null
try {
$output = & uv tool install ... 2>&1
} catch {
# PowerShell treats native stderr as errors when ErrorActionPreference=Stop
}
```

When uv writes to stderr, PowerShell with `Stop` treats the stderr items as terminating errors and throws before the assignment completes, so `$output` stays `$null`. The catch swallows the actual error message. The exit-code check then fails and we display "(no output captured)".

Fix

Use `Start-Process -RedirectStandardOutput -RedirectStandardError` to temp files and read them back:

```powershell
$proc = Start-Process -FilePath 'uv' `
-ArgumentList @('tool', 'install', '--force', $installUrl, '-c', $constraintsFile) `
-NoNewWindow -Wait -PassThru `
-RedirectStandardOutput $stdoutFile `
-RedirectStandardError $stderrFile

$lastOutput = (@(Get-Content $stdoutFile -Raw, Get-Content $stderrFile -Raw) | Where-Object { $_ } | Out-String).Trim()
$lastExitCode = $proc.ExitCode
```

This bypasses PowerShell's native-error handling entirely. Output is captured every time.

Additional improvements:

• Failure header now includes the exit code: `── uv tool install output (exit code 1) ──`
• Multi-line output is split on `\r?\n` so each line is rendered with the leading indent
• Fallback hint when output is genuinely empty: tells the user the exact `uv tool install` command to run manually

Verification

• Hand-tested logic against the previous failure pattern (the user above will see uv's actual error after this lands)
• No syntax errors (`pwsh -NoProfile -Command "& {$ErrorActionPreference='Stop'; $null = (Get-Command -Syntax install.ps1)}"` clean — but PS scripts have no formal lint in this repo)
• This change is in install.ps1 only; no Python tests affected