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")))
 }

crates/codegen/kigi-fast-worktree/src/db/mod.rs

@@ -341,12 +341,14 @@ pub fn resolve_kigi_home() -> Result<PathBuf> {
     if let Ok(v) = std::env::var("KIGI_SHARE_DIR") {
         return Ok(PathBuf::from(v));
     }
-    let home =
-        PathBuf::from(std::env::var("HOME").context("neither $KIGI_SHARE_DIR nor $HOME is set")?);
-    // Canonicalize the home dir so worktree paths share the same physical .kigi
-    // tree as trust/hooks even when it is symlinked. The dunce canonicalization
-    // must stay in sync with kigi_config::default_kigi_home();
-    // home resolution deliberately differs ($HOME here vs std::env::home_dir()).
+    // Standard home-dir lookup (`USERPROFILE`/Known Folders on Windows, where
+    // `HOME` is usually unset outside Git Bash) — the same resolution as
+    // kigi_config::default_kigi_home(), so worktree paths land in the same
+    // physical .kigi tree as trust/hooks.
+    let home = std::env::home_dir()
+        .context("neither $KIGI_SHARE_DIR nor a home directory could be resolved")?;
+    // Canonicalize so a symlinked home still maps to one physical tree; the
+    // dunce canonicalization must stay in sync with default_kigi_home().
     Ok(dunce::canonicalize(&home).unwrap_or(home).join(".kigi"))
 }