fix(tools): find gh across common install paths

## Summary
- Probe common Linux and macOS gh CLI paths when DEEPSEEK_GH_BIN is not set.
- Keep the existing /opt/homebrew/bin/gh fallback for compatibility.
- Improve the not-installed error message.

## Verification
- GitHub CI passed: lint, version drift, ubuntu/macos/windows tests, npm wrapper smoke, GitGuardian.

crates/tui/src/tools/github.rs

@@ -15,6 +15,12 @@ use crate::tools::spec::{
 };
 
 const DEFAULT_GH: &str = "/opt/homebrew/bin/gh";
+const FALLBACK_GH_PATHS: &[&str] = &[
+    "/usr/bin/gh",                       // Linux system package manager
+    "/usr/local/bin/gh",                 // macOS Intel Homebrew / manual install
+    "/home/linuxbrew/.linuxbrew/bin/gh", // Linux Homebrew (official prefix)
+    "/opt/homebrew/bin/gh",              // macOS Apple Silicon Homebrew
+];
 const BODY_ARTIFACT_THRESHOLD: usize = 4_000;
 const DIFF_ARTIFACT_THRESHOLD: usize = 8_000;
 
@@ -300,7 +306,15 @@ impl ToolSpec for GithubCloseIssueTool {
 }
 
 fn gh_bin() -> String {
-    std::env::var("DEEPSEEK_GH_BIN").unwrap_or_else(|_| DEFAULT_GH.to_string())
+    if let Ok(bin) = std::env::var("DEEPSEEK_GH_BIN") {
+        return bin;
+    }
+    for path in FALLBACK_GH_PATHS {
+        if std::path::Path::new(path).is_file() {
+            return path.to_string();
+        }
+    }
+    DEFAULT_GH.to_string()
 }
 
 fn run_gh_text(context: &ToolContext, args: &[&str]) -> Result<String, ToolError> {
@@ -310,7 +324,7 @@ fn run_gh_text(context: &ToolContext, args: &[&str]) -> Result<String, ToolError
         .output()
         .map_err(|e| {
             if e.kind() == std::io::ErrorKind::NotFound {
-                ToolError::not_available("gh CLI is not installed at /opt/homebrew/bin/gh")
+                ToolError::not_available("gh CLI not found; install it or set DEEPSEEK_GH_BIN")
             } else {
                 ToolError::execution_failed(format!("failed to run gh: {e}"))
             }