docs(action): add missing defaults and fix itemId output description

[kranthipoturaju]

Summary

Fixes three inaccuracies in action.yml where the metadata diverged from the actual implementation.

Changes

• labeled: added default: '' — reflects core.getInput('labeled') returning an empty string when not provided
• label-operator: added default: 'OR' — reflects the else branch in the implementation, which applies OR semantics when the input is empty or unset
• itemId output: replaced vague description with one that documents both code paths: linked project item (same owner) and draft issue (cross-owner fallback)
• Top-level description: expanded abbreviation "PRs" → "pull requests" for consistency

Motivation

The labeled and label-operator inputs had no default: fields, so tooling (e.g. the Actions marketplace, editor extensions) could not surface defaults to users without reading the source. The itemId description was
misleading for the cross-owner case, where the action silently creates a draft issue rather than linking the original item.

Impact

• No behavior change — defaults match what the code already does
• Fully backward compatible — no input/output names, types, or required flags changed
• Editors and the Actions marketplace will now display accurate defaults for both optional inputs

Testing

action.yml is metadata-only. Changes verified by inspecting the diff against the implementation in src/add-to-project.ts.

[Copilot]

Pull request overview

This PR updates the action metadata (action.yml) to accurately reflect the action’s existing runtime behavior for optional inputs and the itemId output, improving what users see in the Actions marketplace and editor tooling.

Changes:

• Documents defaults for optional inputs: labeled defaults to an empty string and label-operator defaults to OR.
• Clarifies the itemId output description to cover both “linked project item” and “cross-owner draft issue” paths.
• Expands the top-level description to use “pull requests” instead of “PRs”.

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