fix: spawn fails with bun not found after install (#1748)

* fix: add ~/.bun/bin to shell rc files so spawn finds bun after install

The install script was only adding ~/.local/bin to shell profile files
(bashrc/zshrc/bash_profile), but not ~/.bun/bin. Since the spawn binary
uses #!/usr/bin/env bun as its shebang, bun must be in PATH for spawn
to work. After exec $SHELL, only dirs in rc files are available.

Now ensure_in_path() patches shell rc files for both ~/.local/bin (for
spawn) and ~/.bun/bin (for bun), and correctly checks both when deciding
whether to show "Run spawn" vs "exec $SHELL" instructions.

Fixes #1747

Agent: code-health
Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

* fix: quote dir in fish_add_path to prevent command injection

Address security review feedback on PR #1748 — unquoted ${dir} in
fish command string could allow injection if HOME/BUN_INSTALL env
vars contain metacharacters.

Agent: code-health
Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

---------

Co-authored-by: B <6723574+louisgv@users.noreply.github.com>
Co-authored-by: Claude Sonnet 4.5 <noreply@anthropic.com>

cli/package.json

@@ -1,6 +1,6 @@
 {
   "name": "@openrouter/spawn",
-  "version": "0.6.13",
+  "version": "0.6.14",
   "type": "module",
   "bin": {
     "spawn": "cli.js"