fix(windows): resolve the home directory via the standard lookup

Port of upstream grok-build 1.0.4/1.0.5. Two paths read $HOME only,
which is usually unset on Windows outside Git Bash:

- the global-gitignore fallback (prompt/ignore.rs) now uses
  std::env::home_dir(), the crate-wide idiom;
- fast-worktree's resolve_kigi_home falls back to std::env::home_dir()
  with the same dunce canonicalization, eliminating the documented
  divergence from kigi_config::default_kigi_home. On Unix home_dir()
  reads $HOME first, so behavior is unchanged where HOME is set.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01KSy9S31pZNWXtRZNVJ1sDf

crates/codegen/kigi-agent/src/prompt/ignore.rs

@@ -32,5 +32,5 @@ fn get_global_gitignore_path() -> Option<PathBuf> {
     git2::Config::open_default()
         .ok()
         .and_then(|cfg| cfg.get_path("core.excludesFile").ok())
-        .or_else(|| std::env::var_os("HOME").map(|h| PathBuf::from(h).join(".gitignore")))
+        .or_else(|| std::env::home_dir().map(|h| h.join(".gitignore")))
 }