fix(install): add Set-StrictMode to install.ps1

[MANVENDRA-github]

The repo violates its own rule

rules/powershell/powershell-strict-mode.md — shipped by this repo, synced to four harnesses — opens its "What you do not do" list with:

Ship a .ps1 without Set-StrictMode -Version Latest and $ErrorActionPreference = 'Stop'.

$ grep -c 'Set-StrictMode'        scripts/install.ps1   →  0
$ grep -c 'ErrorActionPreference' scripts/install.ps1   →  1

It had the second and not the first. Meanwhile scripts/install.sh carries set -euo pipefail and quotes every expansion — the shell installer was compliant, the PowerShell one was not.

Why it matters here specifically

install.ps1 resolves destination paths from variables and then removes and copies directories under them:

$targetPath = Join-Path $destSub $entry.Name
if (Test-Path -LiteralPath $targetPath) {
    Remove-Item -LiteralPath $targetPath -Recurse -Force
}

Without strict mode a misspelled variable expands to $null instead of erroring — and a $null path is precisely what those Remove-Item calls must never receive. The existing guards (Test-Path, -LiteralPath, Join-Path) are all correct; strict mode is the one that catches the typo before it reaches them. That is the rule's own argument, quoted back at the script.

Behavior-preserving — verified, not assumed

Strict mode turns a non-existent property access into a terminating error, so this could plausibly break the script. It doesn't. Both the original and the modified script were run from the same path with the same arguments in a throwaway worktree — the only difference being the one added line:

             target     exit
no-strict    claude      0
no-strict    opencode    0
STRICT       claude      0
STRICT       opencode    0

Full matrix on the modified script: -Target claude|cursor|codex|opencode with -Project, then again with -Uninstall — all eight runs exit 0, no VariableIsUndefined / PropertyNotFoundStrict errors. -Help exits 0; a missing -Target still errors with exit 1.

Verification

$ npm run sync   # clean, no drift
$ npm run lint   # PASS
$ npm test       # 239 pass, 0 fail

install.ps1 is not synced into any harness output, so no generated file changes.