fix(install): strip non-ASCII bytes from install.ps1 (#1523)

Follow-up to this PR's first commit. The Windows CI 'apm self-update'
test (Test 5) downloads install.ps1 via Python urllib in the v0.13.0
installed CLI and prints it through a cp1252 console; the script
must therefore be pure ASCII or the print raises
'charmap codec can't encode characters in position 33941-33942'.

Two violations of the repo's ASCII-only rule
(.github/instructions/encoding.instructions.md):

1. Em-dash (U+2014) in a pre-existing comment at byte 29170
   ('need a retry -- acceptable for an install/self-update').
   Replaced with the ASCII '--' digraph.

2. Replacement characters (U+FFFD U+FFFD) at byte 33139 inside the
   comment this PR just added describing what a UTF-16 cmd.exe parse
   failure looks like. I had literally pasted the garbled-output
   illustration verbatim. Reworded to describe the failure mode in
   plain ASCII prose.

install.ps1 now contains zero bytes above 0x7E. The previously-failing
self-update path can write the script to a cp1252 console without
raising UnicodeEncodeError.

Co-authored-by: danielmeppiel <danielmeppiel@users.noreply.github.com>
Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: danielmeppiel

install.ps1

@@ -729,7 +729,7 @@ try {
     # there is still a small gap where neither path exists; doing it this
     # way minimizes that gap and lets us roll back if the second rename
     # fails. Concurrent apm invocations during that window will fail and
-    # need a retry — acceptable for an install/self-update operation.
+    # need a retry -- acceptable for an install/self-update operation.
     $backupDir = $null
     if (Test-Path $releaseDir) {
         $backupDir = "$releaseDir.old-" + [System.Guid]::NewGuid().ToString("N")
@@ -802,7 +802,8 @@ try {
     # Write the shim as ASCII. cmd.exe interprets .cmd files via the system
     # OEM/ANSI code page and does NOT reliably auto-detect UTF-16LE (even
     # with a BOM) when batch files are invoked via PATH or double-click; a
-    # UTF-16 shim surfaces as garbled output like ">��@" and exit code 1.
+    # UTF-16 shim surfaces as garbled bytes (the cmd.exe prompt followed
+    # by replacement-character noise) and exit code 1.
     # ASCII is safe for our payload because the %LOCALAPPDATA% literal
     # token (issue #1509) keeps the embedded shim target ASCII-only even
     # when the user's profile directory contains non-ASCII characters.