fix(wizard): write PATH to .profile/.zshenv, never .bashrc

getShellProfilePath() preferred ~/.bashrc when it existed, and ~/.zshrc for
zsh. setupPath() in stages/finalize.ts appends the PATH export to whatever
it returns. Debian's default ~/.bashrc opens with

    case $- in *i*) ;; *) return;; esac

so a line appended to the bottom of it never runs for 'bash -lc', for
systemd units, for 'ssh host cmd', or for any agent seat — precisely the
consumers that need the CLI. An install could print its summary and exit 0
while leaving 'mosaic: command not found'. .zshrc has the same problem:
zsh only reads it for interactive shells.

Now ~/.profile, which login shells read and which Debian's copy sources
.bashrc from for interactive shells, so one line covers both. For zsh the
always-sourced file is .zshenv. fish and PowerShell are unchanged.

__tests__/platform/detect.test.ts pins it, including a case asserting that
no shell resolves to an interactive-only rc file. Falsified by inverting
the fix: 5 failed / 1 passed; restored 6/6. Full package suite unchanged at
17 files / 4 tests failing, matching clean origin/next.

packages/mosaic/src/platform/detect.ts

@@ -1,4 +1,3 @@
-import { existsSync } from 'node:fs';
 import { join } from 'node:path';
 import { homedir, platform } from 'node:os';
 
@@ -22,15 +21,18 @@ export function getShellProfilePath(): string | null {
 
   const shell = detectShell();
   switch (shell) {
+    // Both of these deliberately avoid the interactive-only rc files.
+    // Debian's default .bashrc returns early for non-interactive shells, so a
+    // PATH line appended to it never runs for `bash -lc`, systemd units, or
+    // agent seats — an install could report success and still leave `mosaic`
+    // unreachable. .profile is read by login shells and sources .bashrc for
+    // interactive ones, so one line covers both; .zshenv is zsh's equivalent.
     case 'zsh': {
       const zdotdir = process.env['ZDOTDIR'] ?? home;
-      return join(zdotdir, '.zshrc');
+      return join(zdotdir, '.zshenv');
     }
-    case 'bash': {
-      const bashrc = join(home, '.bashrc');
-      if (existsSync(bashrc)) return bashrc;
+    case 'bash':
       return join(home, '.profile');
-    }
     case 'fish':
       return join(home, '.config', 'fish', 'config.fish');
     default: