Target place for backlog

[h1alexbel]

closes #96

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PR-Codex overview

This PR focuses on adding support for Backlog in the Tracehub project. It includes the following changes:

• Added Backlog interface
• Implemented YmlBacklog class for YAML-based backlogs
• Updated Project interface to include backlog() method
• Updated relevant test classes to include tests for backlog functionality

The following files were skipped due to too many changes: src/main/java/git/tracehub/facts/ExecOn.java, src/test/java/git/tracehub/agents/YmlBacklogTest.java, src/main/java/git/tracehub/agents/github/GhOrder.java, src/test/java/git/tracehub/facts/ExecOnTest.java, src/main/java/git/tracehub/tk/TkGitHub.java, src/test/java/git/tracehub/agents/github/GhOrderTest.java

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[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (3f1c4bb) 80.58% compared to head (81e3d68) 81.35%. Report is 15 commits behind head on master.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #103 +/- ## ============================================ + Coverage 80.58% 81.35% +0.76% - Complexity 96 104 +8 ============================================ Files 32 35 +3 Lines 340 354 +14 Branches 19 19 ============================================ + Hits 274 288 +14 Misses 59 59 Partials 7 7 ```

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[h1alexbel]

@rultor merge

[h1alexbel]

@rultor merge

[h1alexbel]

@rultor merge

[h1alexbel]

@rultor status