Installers persist PATH themselves instead of printing instructions

install.sh: append the export line to the login shell's rc file
(zsh: $ZDOTDIR/.zshrc; bash: ~/.bash_profile on macOS, ~/.bashrc on
Linux; fish: fish_add_path in config.fish; otherwise ~/.profile).
Idempotent — a second run detects the existing entry and skips. On an
unwritable rc it fails loudly with the manual command.

install.ps1: prepend the bin dir to the per-user PATH via
[Environment]::SetEnvironmentVariable(..., 'User') — registry-backed,
picked up by every new terminal. Strict-mode-safe for a null user Path.

Verified end-to-end on macOS with a fake HOME: install → rc appended →
fresh zsh sources it → kigi resolves and runs. All shell branches,
ZDOTDIR/XDG overrides, idempotency, and the write-failure path covered
by a harness driving the real script tail; install.ps1 parse-checked
and its path-construction logic exercised under pwsh strict mode.

install.ps1

@@ -138,12 +138,13 @@ try {
     $OnPath = (($UserPath -split ";") -contains $BinDir) -or
               (($env:Path -split ";") -contains $BinDir)
     if (-not $OnPath) {
+        # Persist the bin dir on the per-user PATH so the user doesn't have
+        # to. Registry-backed; every new terminal picks it up automatically.
+        $NewUserPath = if ($UserPath) { "$BinDir;$UserPath" } else { $BinDir }
+        [Environment]::SetEnvironmentVariable("Path", $NewUserPath, "User")
         Write-Host ""
-        Write-Host "$BinDir is not on your PATH. Add it for the current user with:"
-        Write-Host ""
-        Write-Host "  [Environment]::SetEnvironmentVariable('Path', `"$BinDir;`" + [Environment]::GetEnvironmentVariable('Path', 'User'), 'User')"
-        Write-Host ""
-        Write-Host "Then open a new terminal and run 'kigi' to get started."
+        Write-Host "Added $BinDir to your user PATH."
+        Write-Host "Open a new terminal, then run 'kigi' to get started."
     } else {
         Write-Host "Run 'kigi' to get started."
     }