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>

lib/src/panes/registry.dart

@@ -11,6 +11,7 @@ import 'dart:convert';
 import 'dart:io' show Platform;
 import 'dart:typed_data';
 
+import '../env/shell_env.dart' show resolvedToolPath;
 import '../ipc/envelope.dart';
 import '../pty/pty_log.dart';
 import '../pty/pty_session.dart';
@@ -53,6 +54,9 @@ class PaneRegistry {
     // Terminal defaults for the PTY child.
     final fullEnv = <String, String>{
       ...Platform.environment,
+      // The login-shell-resolved PATH so PTY children find user-installed tools
+      // even on a desktop launch (T-439); an explicit caller PATH still wins.
+      'PATH': resolvedToolPath(),
       'TERM': 'xterm-256color',
       'COLORTERM': 'truecolor',
       'LANG': 'en_US.UTF-8',