Update tracker with existing contributions

Add section documenting 3 merged PRs (dmlc#12087, dmlc#12089, dmlc#12094),
1 open PR (dmlc#12086), and 3 closed/superseded PRs.

Author: PavelGuzenfeld

doc/contrib/contribution-tracker.md

@@ -1,12 +1,47 @@
 # XGBoost Upstream Contribution Research & Progress
 
-**Last updated:** 2026-03-19
+**Last updated:** 2026-03-20
 **Upstream repo:** [dmlc/xgboost](https://github.com/dmlc/xgboost)
+**Fork:** [PavelGuzenfeld/xgboost](https://github.com/PavelGuzenfeld/xgboost)
 **License:** Apache-2.0
 
 ---
 
-## Easy Entry Points
+## My Contributions
+
+### Merged
+
+- [x] [PR #12087](https://github.com/dmlc/xgboost/pull/12087) — **[doc] Update competition winning solutions list with recent entries**
+  - Updated `demo/README.md` with recent Kaggle wins
+  - Merged: 2026-03-15
+
+- [x] [PR #12089](https://github.com/dmlc/xgboost/pull/12089) — **[python-package] Handle indicator (boolean) features in `trees_to_dataframe`**
+  - Fixed handling of boolean/indicator features in tree dataframe output
+  - Merged: 2026-03-15
+
+- [x] [PR #12094](https://github.com/dmlc/xgboost/pull/12094) — **[python-package] Fix python -OO crash by guarding `__doc__` assignments**
+  - Guarded docstring assignments that crashed under `python -OO` (optimized mode)
+  - Merged: 2026-03-16
+
+### Open
+
+- [ ] [PR #12086](https://github.com/dmlc/xgboost/pull/12086) — **[python-package] Remove dead Python 2 guard and `sklearn.cross_validation` fallback**
+  - Removed unreachable legacy compat code (Python 2 guard, old sklearn import)
+  - Approved by RAMitchell (core maintainer)
+  - CI: all green (1 macOS wheel build flaky failure — not related)
+  - **Status:** Awaiting merge
+
+### Closed (superseded)
+
+- [PR #12085](https://github.com/dmlc/xgboost/pull/12085) — Superseded by #12094 (fix python -OO crash)
+- [PR #12088](https://github.com/dmlc/xgboost/pull/12088) — Closed: booster type mismatch warning
+- [PR #12093](https://github.com/dmlc/xgboost/pull/12093) — Superseded by #12094 (docstring guard)
+
+---
+
+## Contribution Opportunities
+
+### Easy Entry Points
 
 - [ ] [#11413](https://github.com/dmlc/xgboost/issues/11413) — **Port tutorials to R**
   - 25 Python-only tutorials need R tabs (Sphinx mechanism already exists)
@@ -17,9 +52,9 @@
   - Codebase has grown significantly, type annotations increasingly important
   - **Status:**
 
-- [ ] [#6173](https://github.com/dmlc/xgboost/issues/6173) — **Update competition winning solutions**
-  - `demo/README.md` list stale since 2017; a PR merged 2026-03-15 but more to add
-  - **Status:**
+- [x] [#6173](https://github.com/dmlc/xgboost/issues/6173) — **Update competition winning solutions**
+  - `demo/README.md` list stale since 2017
+  - **Status:** Contributed via [PR #12087](https://github.com/dmlc/xgboost/pull/12087) (merged 2026-03-15). More entries may still be added.
 
 ---
 
@@ -113,4 +148,8 @@ _Use this section to track which items you've decided to pursue, discussions wit
 
 | Date | Action |
 |------|--------|
-| 2026-03-19 | Initial research completed — catalogued all contribution opportunities |
+| 2026-03-15 | PR #12087 merged — updated competition winning solutions (issue #6173) |
+| 2026-03-15 | PR #12089 merged — fixed boolean features in `trees_to_dataframe` |
+| 2026-03-15 | PR #12086 opened — remove dead Python 2 compat code (approved, awaiting merge) |
+| 2026-03-16 | PR #12094 merged — fixed python -OO crash with `__doc__` guards |
+| 2026-03-20 | Initial research completed — catalogued all contribution opportunities |