fix: use positional argument instead of deprecated --prompt flag

The Gemini CLI has deprecated the --prompt (-p) flag and recommends
using positional arguments instead. This change updates the CLI
invocation to pass the prompt as a positional argument, eliminating
the deprecation warning:

  "The --prompt (-p) flag has been deprecated and will be removed
  in a future version. Please use a positional argument for your
  prompt."

Changes:
- Update gemini CLI invocations to use positional argument
- Update input description to reflect positional argument usage
- Regenerate README.md documentation

Fixes #453

Author: sempi

README.md

@@ -175,7 +175,7 @@ go to the [Gemini Assistant workflow documentation](./examples/workflows/gemini-
 
 -   <a name="__input_google_api_key"></a><a href="#user-content-__input_google_api_key"><code>google_api_key</code></a>: _(Optional)_ The Vertex AI API key to use with Gemini.
 
--   <a name="__input_prompt"></a><a href="#user-content-__input_prompt"><code>prompt</code></a>: _(Optional, default: `You are a helpful assistant.`)_ A string passed to the Gemini CLI's [`--prompt` argument](https://github.com/google-gemini/gemini-cli/blob/main/docs/get-started/configuration.md#command-line-arguments).
+-   <a name="__input_prompt"></a><a href="#user-content-__input_prompt"><code>prompt</code></a>: _(Optional, default: `You are a helpful assistant.`)_ A string passed to the Gemini CLI as a [positional argument](https://github.com/google-gemini/gemini-cli/blob/main/docs/get-started/configuration.md#command-line-arguments).
 
 -   <a name="__input_settings"></a><a href="#user-content-__input_settings"><code>settings</code></a>: _(Optional)_ A JSON string written to `.gemini/settings.json` to configure the CLI's _project_ settings.
     For more details, see the documentation on [settings files](https://github.com/google-gemini/gemini-cli/blob/main/docs/get-started/configuration.md#settings-files).

action.yml

@@ -56,7 +56,7 @@ inputs:
     required: false
   prompt:
     description: |-
-      A string passed to the Gemini CLI's [`--prompt` argument](https://github.com/google-gemini/gemini-cli/blob/main/docs/get-started/configuration.md#command-line-arguments).
+      A string passed to the Gemini CLI as a [positional argument](https://github.com/google-gemini/gemini-cli/blob/main/docs/get-started/configuration.md#command-line-arguments).
     required: false
     default: 'You are a helpful assistant.'
   settings:
@@ -294,14 +294,14 @@ runs:
         if [[ "${GEMINI_DEBUG}" = true ]]; then
           echo "::warning::Gemini CLI debug logging is enabled. This will stream responses, which could reveal sensitive information if processed with untrusted inputs."
           echo "::: Start Gemini CLI STDOUT :::"
-          if ! gemini --debug --yolo --prompt "${PROMPT}" --output-format json 2> >(tee "${TEMP_STDERR}" >&2) | tee "${TEMP_STDOUT}"; then
+          if ! gemini --debug --yolo --output-format json "${PROMPT}" 2> >(tee "${TEMP_STDERR}" >&2) | tee "${TEMP_STDOUT}"; then
             FAILED=true
           fi
           # Wait for async stderr logging to complete. This is because process substitution in Bash is async so let tee finish writing to ${TEMP_STDERR}
           sleep 1
           echo "::: End Gemini CLI STDOUT :::"
         else
-          if ! gemini --yolo --prompt "${PROMPT}" --output-format json 2> "${TEMP_STDERR}" 1> "${TEMP_STDOUT}"; then
+          if ! gemini --yolo --output-format json "${PROMPT}" 2> "${TEMP_STDERR}" 1> "${TEMP_STDOUT}"; then
             FAILED=true
           fi
         fi