feat: add session_id output to enable resuming conversations (#739)

Add a new `session_id` output that exposes the Claude Code session ID,
allowing other workflows or Claude Code instances to resume the
conversation using `--resume <session_id>`.

Changes:
- Add parseAndSetSessionId() function to extract session_id from
  the system.init message in execution output
- Add session_id output to both action.yml and base-action/action.yml
- Add comprehensive tests for the new functionality

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-authored-by: Claude Opus 4.5 <noreply@anthropic.com>

action.yml

@@ -127,6 +127,9 @@ outputs:
   structured_output:
     description: "JSON string containing all structured output fields when --json-schema is provided in claude_args. Use fromJSON() to parse: fromJSON(steps.id.outputs.structured_output).field_name"
     value: ${{ steps.claude-code.outputs.structured_output }}
+  session_id:
+    description: "The Claude Code session ID that can be used with --resume to continue this conversation"
+    value: ${{ steps.claude-code.outputs.session_id }}
 
 runs:
   using: "composite"