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/kernel/src/toolchain_paths.dart

@@ -10,6 +10,13 @@ library;
 
 import 'dart:io';
 
+import 'package:clide/src/env/shell_env.dart';
+
+// The canonical PATH-expansion logic now lives in shell_env (T-439, the single
+// source of truth shared with git/pql/PTY/claude). Re-exported so existing
+// importers/tests keep resolving it from here.
+export 'package:clide/src/env/shell_env.dart' show expandToolPath;
+
 /// Serializable result of tool resolution (crosses isolate boundary).
 class ResolvedPaths {
   const ResolvedPaths({this.git, this.pql, this.shell, this.gitEnv});
@@ -144,25 +151,8 @@ String? _firstExisting(List<String> candidates) {
   return null;
 }
 
-/// Build expanded PATH inline — must be self-contained for isolate use.
-String _expandedPath() =>
-    expandToolPath(Platform.environment['PATH'] ?? '', isMac: Platform.isMacOS, isLinux: Platform.isLinux, home: Platform.environment['HOME']);
-
-/// Pure PATH-expansion logic, extracted so it's testable without touching the
-/// process environment.
-///
-/// A desktop-launched app (macOS or Linux) inherits a minimal PATH that lacks
-/// the user bin dirs where tools like `pql` install (`~/.local/bin`), so tool
-/// resolution fails even though a terminal launch would find them. Re-add the
-/// common user/local bin dirs — that any are missing means they're prepended,
-/// so they take precedence over a stale system copy (T-347). Homebrew dirs are
-/// macOS-only. On other platforms the base PATH passes through unchanged.
-String expandToolPath(String base, {required bool isMac, required bool isLinux, String? home}) {
-  if (!isMac && !isLinux) return base;
-  final h = home ?? '';
-  final extras = <String>[if (h.isNotEmpty) '$h/.local/bin', if (isMac) '/opt/homebrew/bin', if (isMac) '/opt/homebrew/sbin', '/usr/local/bin'];
-  final existing = base.split(':').toSet();
-  final missing = extras.where((p) => !existing.contains(p));
-  if (missing.isEmpty) return base;
-  return [...missing, ...existing].join(':');
-}
+/// The full tool search PATH — the login-shell PATH (probed once at startup)
+/// unioned with the well-known user/local bin dirs, shared with every other
+/// spawn site via [shell_env] (T-439). In an isolate that never primed the
+/// probe it degrades to the process PATH + the well-known dirs (T-347).
+String _expandedPath() => resolvedToolPath();