feat(installer): add Claude CLI preflight checks (found, version, login, auto-mode)

Four IEnvironmentCheck implementations in src/ClaudeDo.Installer/Checks/:
- ClaudeCliCheck (Error) — resolves ctx.ClaudeBin via ExecutableResolver, runs
  --version; failure message lists searched PATH entries + fallback dirs, flags
  shim resolution (.cmd/.ps1) in Detail.
- ClaudeVersionCheck (Error) — tolerant version parsing (ignores surrounding
  text), numeric System.Version comparison against a named floor constant
  (2.1.220, see docs/explore-notes/installer-preflight.md §3). Unparseable or
  missing CLI -> Unknown, never Failed.
- ClaudeAuthCheck (Error) — `claude auth status --json`, never sends a prompt.
- PermissionModeAutoCheck (Warning) — per the note's §2 conclusion, real
  auto-mode eligibility (org/model/plan) has no cheap static signal, so this
  only confirms `claude --help` still lists "auto" as a --permission-mode
  choice. Kept as its own check rather than folded into ClaudeVersionCheck,
  since the note explicitly separates "flag recognized" from "mode eligible"
  and only the former is checkable at all.

All four share a new ClaudeCliLookup that resolves the CLI and runs
--version exactly once per install run (memoized, semaphore-guarded), so
none of the three version-dependent checks repeats that call.

Foundation prerequisites this task depends on (docs/explore-notes/
installer-preflight.md, ExecutableResolver, the IEnvironmentCheck/CheckResult/
EnvironmentCheckService abstraction, ProcessCommand, IProcessRunner) existed
only on an unmerged sibling branch, not on main. Rather than merging that
whole unreviewed branch, the specific prerequisite files were copied in
as-is (git history shows their origin). GitCheck/GitIdentityCheck/PortCheck/
WriteAccessCheck from that branch were intentionally NOT brought in — out of
scope for this task.

Deviations/decisions worth flagging:
- Added ExecutableResolver.FallbackDirectories() (public) so ClaudeCliCheck
  can name the checked fallback locations in its failure message; the prior
  branch kept that list private.
- Paths.cs now qualifies System.Environment explicitly. Adding the
  ClaudeDo.Data.Environment namespace makes bare `Environment.X` inside any
  ClaudeDo.Data.* namespace resolve to the sibling namespace instead of
  System.Environment (C# prefers nested/enclosing namespace members over
  usings) — this broke the build until qualified.

Not done (explicitly out of scope): no DI wiring into the wizard UI, no
XAML — matches how the prerequisite Git/Port/WriteAccess checks were also
left unwired.

src/ClaudeDo.Data/Environment/ExecutableResolver.cs

@@ -0,0 +1,116 @@
+using SysEnvironment = System.Environment;
+
+namespace ClaudeDo.Data.Environment;
+
+public sealed record ResolvedExecutable(string Path, bool IsShim);
+
+public sealed record ShimStartInfo(string FileName, string Arguments);
+
+/// <summary>
+/// Resolves a command the way Windows' CreateProcess/PATH search does, but also finds
+/// non-.exe shims (.cmd/.bat/.ps1) that UseShellExecute=false alone would miss.
+/// </summary>
+public static class ExecutableResolver
+{
+    private const string DefaultPathExt = ".COM;.EXE;.BAT;.CMD";
+
+    // Known npm/claude install locations to try when PATH search comes up empty.
+    private static readonly string[] FallbackDirectoryTemplates =
+    {
+        "%APPDATA%\\npm",
+        "%LOCALAPPDATA%\\Programs\\claude",
+        "%USERPROFILE%\\.local\\bin",
+    };
+
+    public static ResolvedExecutable? Resolve(string command, string? pathOverride = null, string? pathExtOverride = null)
+    {
+        var pathExts = ParsePathExt(pathExtOverride);
+
+        if (LooksLikePath(command))
+        {
+            return ResolveAsPath(command, pathExts);
+        }
+
+        var directories = ParsePath(pathOverride);
+        foreach (var dir in directories)
+        {
+            var resolved = ResolveInDirectory(dir, command, pathExts);
+            if (resolved is not null) return resolved;
+        }
+
+        foreach (var template in FallbackDirectoryTemplates)
+        {
+            var dir = SysEnvironment.ExpandEnvironmentVariables(template);
+            var resolved = ResolveInDirectory(dir, command, pathExts);
+            if (resolved is not null) return resolved;
+        }
+
+        return null;
+    }
+
+    /// <summary>Expanded fallback directories tried when PATH search comes up empty (for diagnostics).</summary>
+    public static IReadOnlyList<string> FallbackDirectories() =>
+        FallbackDirectoryTemplates.Select(SysEnvironment.ExpandEnvironmentVariables).ToList();
+
+    public static ShimStartInfo BuildShimStartInfo(string shimPath, IReadOnlyList<string> arguments)
+    {
+        var parts = new List<string> { "/c", Quote(shimPath) };
+        parts.AddRange(arguments.Select(Quote));
+        return new ShimStartInfo("cmd.exe", string.Join(' ', parts));
+    }
+
+    private static bool LooksLikePath(string command) =>
+        command.Contains(Path.DirectorySeparatorChar) || command.Contains(Path.AltDirectorySeparatorChar);
+
+    private static ResolvedExecutable? ResolveAsPath(string command, IReadOnlyList<string> pathExts)
+    {
+        if (File.Exists(command)) return new ResolvedExecutable(command, IsShimExtension(Path.GetExtension(command)));
+
+        if (Path.HasExtension(command)) return null;
+
+        foreach (var ext in pathExts)
+        {
+            var candidate = command + ext;
+            if (File.Exists(candidate)) return new ResolvedExecutable(candidate, IsShimExtension(ext));
+        }
+
+        return null;
+    }
+
+    private static ResolvedExecutable? ResolveInDirectory(string directory, string command, IReadOnlyList<string> pathExts)
+    {
+        if (!Directory.Exists(directory)) return null;
+
+        if (Path.HasExtension(command))
+        {
+            var candidate = Path.Combine(directory, command);
+            return File.Exists(candidate) ? new ResolvedExecutable(candidate, IsShimExtension(Path.GetExtension(candidate))) : null;
+        }
+
+        foreach (var ext in pathExts)
+        {
+            var candidate = Path.Combine(directory, command + ext);
+            if (File.Exists(candidate)) return new ResolvedExecutable(candidate, IsShimExtension(ext));
+        }
+
+        return null;
+    }
+
+    private static bool IsShimExtension(string extension) =>
+        !extension.Equals(".exe", StringComparison.OrdinalIgnoreCase)
+        && !extension.Equals(".com", StringComparison.OrdinalIgnoreCase);
+
+    private static IReadOnlyList<string> ParsePathExt(string? pathExtOverride)
+    {
+        var raw = pathExtOverride ?? SysEnvironment.GetEnvironmentVariable("PATHEXT") ?? DefaultPathExt;
+        return raw.Split(';', StringSplitOptions.RemoveEmptyEntries | StringSplitOptions.TrimEntries);
+    }
+
+    private static IReadOnlyList<string> ParsePath(string? pathOverride)
+    {
+        var raw = pathOverride ?? SysEnvironment.GetEnvironmentVariable("PATH") ?? "";
+        return raw.Split(Path.PathSeparator, StringSplitOptions.RemoveEmptyEntries | StringSplitOptions.TrimEntries);
+    }
+
+    private static string Quote(string value) => value.Contains(' ') ? $"\"{value}\"" : value;
+}