fix(opencode): correct PowerShell 5.1 quoting guidance - #42406

Windows PowerShell 5.1 does not escape double quotes when it builds the
command line for an external program. A double-quoted segment loses its
quotes and can split into several arguments, with no error and a success
exit code, so commands are silently corrupted rather than failing.

The 5.1 shell notes told the agent to "escape special characters with the
PowerShell backtick character". For this failure the backtick makes things
worse rather than better:

  plain             ["a b","c d"]      2 args, quotes gone
  backtick escape   ["a ","b","c d"]   3 args
  backslash escape  ["a \"b c\" d"]    correct
  single quotes     ["a 'b c' d"]      correct

Replace that line with the native argument passing caveat and the two
workarounds that were measured to survive. The backtick is still described
as correct for escaping inside PowerShell's own strings, which it is.

This is a PowerShell defect, not an opencode one. The command reaches
PowerShell intact; the corruption happens on the hop to the child process.
An in-memory PowerShell variable holding the string is still split when
passed to a native program, and -EncodedCommand does not help either, so
opencode cannot escape or encode its way out of it. Fixed upstream in
PowerShell 7.3 by $PSNativeCommandArgumentPassing = 'Standard', which does
not exist in 5.1. Shell.win() already prefers pwsh when it is installed, so
this only affects users without PowerShell 7+.

The pwsh notes are deliberately left unchanged.

Verified on Windows PowerShell 5.1.19041.7663 (Desktop):

  cd packages/opencode && bun test test/tool/shell.test.ts   74 pass, 0 fail
  cd packages/core     && bun test test/shell.test.ts        11 pass, 0 fail
  cd packages/opencode && bun typecheck                      exit 0
  cd packages/core     && bun typecheck                      exit 0