fix(install): cross-platform binary install + PATH conflict detection

install_binary() now:
1. Automatically picks best location per platform:
   - macOS: ~/.cargo/bin/ (no sudo, in PATH for Rust users)
   - Linux: ~/.cargo/bin/ → /usr/local/bin → ~/.local/bin/
   - Windows: /usr/local/bin (Git Bash/MSYS2)
2. Detects if old binary exists at a different PATH location
   and warns the user: 'There is ANOTHER gyandoot at ...'
3. Verifies 'command -v gyandoot' resolves correctly after install
4. Shows shell profile instructions if PATH needs updating

Previous issue: always installed to /usr/local/bin with sudo,
overwriting the old binary from install.sh but not warning if
another copy existed elsewhere in PATH.

install.sh

@@ -260,16 +260,113 @@ install_binary() {
         err "Binary not found at $target_dir/$bin_name"
     fi
 
-    local install_dir="/usr/local/bin"
-    if [ ! -w "$install_dir" ]; then
-        info "Need sudo to install to $install_dir"
-        sudo cp "$target_dir/$bin_name" "$install_dir/$bin_name"
-        sudo chmod +x "$install_dir/$bin_name"
-    else
-        cp "$target_dir/$bin_name" "$install_dir/$bin_name"
-        chmod +x "$install_dir/$bin_name"
+    # ── Detect best install location per platform ─────────────────────
+    # Priority:
+    #   1. ~/.cargo/bin/  — always in PATH for Rust users, no sudo
+    #   2. /usr/local/bin/ — standard Unix location, may need sudo
+    #   3. Windows: use a standard PATH location or just warn
+    #
+    local os
+    os="$(uname -s)"
+
+    local install_dir=""
+    local installed_to=""
+
+    # Detect WSL (Windows Subsystem for Linux)
+    local is_wsl=false
+    if [ -f /proc/version ] && grep -qiE 'microsoft|wsl' /proc/version 2>/dev/null; then
+        is_wsl=true
+    fi
+
+    case "$os" in
+        Darwin|Linux)
+            # Option A: ~/.cargo/bin/ (no sudo needed, always in PATH for Rust users)
+            if [ -d "$HOME/.cargo/bin" ] && echo "$PATH" | grep -q "$HOME/.cargo/bin"; then
+                install_dir="$HOME/.cargo/bin"
+                cp "$target_dir/$bin_name" "$install_dir/$bin_name"
+                chmod +x "$install_dir/$bin_name"
+                installed_to="$install_dir/$bin_name"
+            # Option B: /usr/local/bin/ (standard, may need sudo)
+            elif [ -d "/usr/local/bin" ]; then
+                if [ -w "/usr/local/bin" ]; then
+                    install_dir="/usr/local/bin"
+                    cp "$target_dir/$bin_name" "$install_dir/$bin_name"
+                    chmod +x "$install_dir/$bin_name"
+                else
+                    info "Need sudo to install to /usr/local/bin"
+                    sudo cp "$target_dir/$bin_name" "/usr/local/bin/$bin_name"
+                    sudo chmod +x "/usr/local/bin/$bin_name"
+                    install_dir="/usr/local/bin"
+                fi
+                installed_to="$install_dir/$bin_name"
+            # Option C: ~/.local/bin/ (XDG, common on modern Linux)
+            elif [ -d "$HOME/.local/bin" ] && echo "$PATH" | grep -q "$HOME/.local/bin"; then
+                install_dir="$HOME/.local/bin"
+                cp "$target_dir/$bin_name" "$install_dir/$bin_name"
+                chmod +x "$install_dir/$bin_name"
+                installed_to="$install_dir/$bin_name"
+            else
+                # Fallback: just copy to ~/.cargo/bin and create it
+                mkdir -p "$HOME/.cargo/bin"
+                cp "$target_dir/$bin_name" "$HOME/.cargo/bin/$bin_name"
+                chmod +x "$HOME/.cargo/bin/$bin_name"
+                install_dir="$HOME/.cargo/bin"
+                installed_to="$install_dir/$bin_name"
+                if ! echo "$PATH" | grep -q "$HOME/.cargo/bin"; then
+                    warn "$HOME/.cargo/bin is not in your PATH."
+                    warn "Add this to your shell profile (~/.zshrc, ~/.bashrc, etc.):"
+                    echo "  export PATH=\"\$HOME/.cargo/bin:\$PATH\""
+                fi
+            fi
+            ;;
+        CYGWIN*|MINGW*|MSYS*)
+            # Windows / Git Bash / MSYS2
+            install_dir="/usr/local/bin"
+            cp "$target_dir/$bin_name" "$install_dir/$bin_name"
+            chmod +x "$install_dir/$bin_name"
+            installed_to="$install_dir/$bin_name"
+            warn "Windows detected. Make sure '$install_dir' is in your PATH."
+            ;;
+        *)
+            err "Unsupported OS: $os"
+            ;;
+    esac
+
+    ok "Installed to $installed_to"
+
+    # ── Verify it's usable via PATH ──────────────────────────────────
+    local cmd_path
+    cmd_path="$(command -v gyandoot 2>/dev/null || true)"
+    if [ -z "$cmd_path" ]; then
+        warn "'gyandoot' not found in PATH after install."
+        warn "The install directory may not be in your PATH."
+        warn "Try running with: $installed_to"
+        if [ -n "$install_dir" ]; then
+            case "$os" in
+                Darwin)
+                    echo ""
+                    echo "  Add to your ~/.zshrc:"
+                    echo "    export PATH=\"$install_dir:\$PATH\""
+                    echo "  Then: source ~/.zshrc"
+                    ;;
+                Linux)
+                    echo ""
+                    echo "  Add to your ~/.bashrc:"
+                    echo "    export PATH=\"$install_dir:\$PATH\""
+                    echo "  Then: source ~/.bashrc"
+                    ;;
+            esac
+        fi
+    elif [ "$cmd_path" != "$installed_to" ]; then
+        warn "There's ANOTHER 'gyandoot' at '$cmd_path' that takes priority over '$installed_to'."
+        warn "That binary is from a PREVIOUS installation and may be outdated."
+        warn ""
+        warn "  ▶ To use the NEW version, run: $installed_to"
+        warn "  ▶ Or remove the old one: sudo rm $cmd_path"
+        warn "  ▶ Then 'gyandoot run' will pick up the new one."
+    else
+        ok "PATH resolves to the newly installed binary."
     fi
-    ok "Installed to $install_dir/$bin_name"
 }
 
 print_post_install() {