feature: add only-draft-pr option to only process draft pull requests

[katbyte]

Description:

Adds a new only-draft-pr option that is the inverse of the existing exempt-draft-pr option.

When only-draft-pr: true is set, the action will only process draft pull requests and skip non-draft PRs. This is useful for workflows that want to apply stale handling specifically to draft PRs (e.g., to remind contributors to finish or close abandoned drafts).

Related issue:
closes #1162

Check list:

• Mark if documentation changes are required.
• [x ] Mark if tests were added or updated to cover the changes.

[Copilot]

Pull request overview

Adds a new only-draft-pr option so the stale action can target only draft pull requests, complementing the existing exempt-draft-pr option.

Changes:

• Introduces the onlyDraftPr flag to the processor options, wiring it through action.yml, argument parsing, and the runtime IssuesProcessor.
• Adds an OnlyDraftPullRequest helper that skips non-draft PRs when only-draft-pr is enabled, and integrates it into the processing flow after the existing draft exemption logic.
• Updates documentation and tests (including a dedicated only-draft-pr spec) plus the compiled dist and lockfile to reflect the new option.

Reviewed changes

Copilot reviewed 10 out of 12 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
src/main.ts	Adds onlyDraftPr to the parsed action inputs so the processor receives the new option.
src/interfaces/issues-processor-options.ts	Extends IIssuesProcessorOptions with a boolean onlyDraftPr flag.
src/enums/option.ts	Adds OnlyDraftPr enum value for consistent option name handling and logging.
src/classes/only-draft-pull-request.ts	Implements the OnlyDraftPullRequest helper that skips non-draft PRs when onlyDraftPr is true, logging the decision.
src/classes/issues-processor.ts	Hooks OnlyDraftPullRequest into the processing pipeline after ExemptDraftPullRequest, ending processing early for non-draft PRs when configured.
src/classes/issue.spec.ts	Updates the test options fixture to include the new onlyDraftPr property.
package-lock.json	Regenerates the lockfile, removing several "peer": true markers according to the current npm lockfile format.
dist/index.js	Updates the compiled bundle to include the new option, helper class, and processing logic.
action.yml	Exposes the only-draft-pr input with default false and a short description.
tests/only-draft-pr.spec.ts	Adds tests verifying that non-draft PRs are skipped and draft PRs are processed when only-draft-pr is enabled, and that behavior is unchanged when it is disabled.
tests/constants/default-processor-options.ts	Extends test defaults with onlyDraftPr: false to mirror runtime defaults.
README.md	Documents the only-draft-pr option in the inputs table and options section, describing its behavior as the inverse of exempt-draft-pr.

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