PR–Issue Linking & Label Intelligence (v1.5.0)

### Background

The association model between pull requests and issues is a core engine of the action. Currently, linking relies on explicit references only, and label resolution does not consider both entities together. This leads to PRs or issues being misclassified in the generated release notes.

This epic improves the PR–Issue association model with a weak-link mechanism, merges label logic across both entities, and enables the tool to use rich issue-body release notes in place of issue titles.

### Goals

- Introduce a flexible, improved PR–Issue linking and association model (#228)
- Implement weak (heuristic) linking between PRs and Issues when explicit links are absent (#79)
- Merge PR and issue labels when determining the release notes group for an entry (#96)
- Allow ignoring the issue title when explicit release notes are provided inside the issue body (#40)
- Maintain backward compatibility with existing explicit-link behaviour

### Subtasks

- [ ] #228 Feature: Improved PR-Issue Linking and Association Model
- [ ] #79 Introduce weak link between PR and Issue
- [ ] #96 Merge PR and issue labels when considering release notes group
- [ ] #40 Release Notes: Ignore Issue Title if there are release notes written within a given ticket

### Related / References

- #228 is the umbrella design issue for this epic