fix(install): default to /usr/local/bin (on PATH for all users)

Root or writable /usr/local/bin: install there (no PATH edits needed).
Non-root without sudo: fall back to ~/.local/bin + auto-add to .bashrc.
This eliminates the 'NOTE: not on your PATH' message for the common case.

SHA256SUMS

@@ -1 +1 @@
-d3e91c4b932a96a31d1ca1b805f24b42d23c6e26e63f2c28d7c7798321eb31fe  orca-v0.12.18-linux-amd64.tar.gz
+1355da96421a15de6ec0d7ac0678f463762b8cad31133fb437243c6bda224b04  orca-v0.12.18-linux-amd64.tar.gz

scripts/install.sh

@@ -69,6 +69,9 @@ if [ "$SYSTEM" = "true" ]; then
   fi
   INSTALL_BIN="/usr/local/bin/orca"
   NAMESPACE_DIR="/root/.orca"
+elif [ -w /usr/local/bin ] || [ "$(id -u)" -eq 0 ]; then
+  INSTALL_BIN="/usr/local/bin/orca"
+  NAMESPACE_DIR="${HOME}/.orca"
 else
   INSTALL_BIN="${HOME}/.local/bin/orca"
   NAMESPACE_DIR="${HOME}/.orca"
@@ -259,9 +262,23 @@ if [ "$SYSTEM" = "true" ]; then
   info "  namespace root: ${NAMESPACE_DIR} (use 'orca --system init' to initialize)"
 else
   info "  namespace root: ${NAMESPACE_DIR} (use 'orca init' to initialize)"
+  # If the install dir is not on PATH, add it to .bashrc automatically.
   if ! echo "$PATH" | grep -q "$INSTALL_DIR"; then
-    info "  NOTE: $INSTALL_DIR is not on your PATH. Add it:"
-    info "    export PATH=\"\$PATH:$INSTALL_DIR\""
+    info "  Adding $INSTALL_DIR to PATH via ~/.bashrc..."
+    SHELL_RC="${HOME}/.bashrc"
+    if [ -f "$SHELL_RC" ]; then
+      # Append only if not already present (idempotent).
+      if ! grep -qF "$INSTALL_DIR" "$SHELL_RC"; then
+        echo "" >> "$SHELL_RC"
+        echo "# Added by orca install.sh" >> "$SHELL_RC"
+        echo "export PATH=\"\$PATH:$INSTALL_DIR\"" >> "$SHELL_RC"
+        info "  ✓ Added 'export PATH=\"\$PATH:$INSTALL_DIR\"' to ~/.bashrc"
+        info "  Run 'source ~/.bashrc' or start a new shell to pick up the change."
+      fi
+    else
+      info "  NOTE: ~/.bashrc not found. Add to PATH manually:"
+      info "    export PATH=\"\$PATH:$INSTALL_DIR\""
+    fi
   fi
 fi