fix(env): one login-shell-derived PATH for every spawned tool (T-439)

Desktop/dock-launched clide inherits a sparse PATH (no ~/.local/bin, brew,
nvm, …), so pql/git/claude and PTY tools went missing. T-347 fixed only the
toolchain/pql path on Linux; env.dart's expander was still macOS-only and
claude/PTY/git used the raw PATH — the breakage recurred per spawn site
because there were three divergent expanders.

Consolidate into one resolver (lib/src/env/shell_env.dart):
- primeLoginShellPath(): probe the user's real login shell once at startup
  (`$SHELL -l -c`, sentinel-framed, bounded timeout, graceful fallback to the
  process PATH). Captures the user's actual PATH, not a hardcoded guess.
- expandToolPath(): the canonical merge (moved from toolchain_paths, which
  re-exports it for its tests) — unions the well-known user/local bin dirs.
- resolvedToolPath(): currentSearchPath() + expandToolPath, the single call
  every spawn site uses.

Routed through it: PTY children (registry.dart now overrides PATH), git
(env.dart → operations.dart), the toolchain probe (toolchain_paths), and
hosted claude (agent_bootstrap). Primed in main.dart's !kIsWeb boot. Deleted
the macOS-only env.dart copy and the cli_install copy.

Tests: new shell_env_test (probe + every fallback + merge); env_test and
cli_install_test updated to the consolidated surface. analyze clean, web wasm
build still green, make test green.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

test/pty/env_test.dart

@@ -23,20 +23,16 @@ void main() {
     });
   });
 
-  group('expandedPath', () {
-    test('returns a non-empty string on every platform', () {
+  group('expandedPath (delegates to the shared resolver, T-439)', () {
+    test('non-empty on every platform', () {
       expect(expandedPath, isNotEmpty);
     });
 
-    test('on Linux/Windows, equals Platform.environment[PATH]', () {
-      if (Platform.isMacOS) return; // macOS path-merge tested separately.
-      expect(expandedPath, Platform.environment['PATH']);
-    });
-
-    test('on macOS, includes the well-known extras', () {
-      if (!Platform.isMacOS) return;
-      // Homebrew is the canonical one; at least one of these should appear.
-      expect(expandedPath, anyOf(contains('/opt/homebrew/bin'), contains('/usr/local/bin')));
+    test('is a superset of the process PATH (never drops entries)', () {
+      final got = expandedPath.split(':').toSet();
+      for (final dir in (Platform.environment['PATH'] ?? '').split(':').where((e) => e.isNotEmpty)) {
+        expect(got, contains(dir));
+      }
     });
   });