chore: Clean up repository for public release

Removed internal development files:
- PHASE3B_COMPLETION.md (internal dev log)
- PHASE4_COMPLETION.md (internal dev log)
- SPRINT4_COMPLETION.md (internal dev log)
- STATISTICS.md (old test statistics)
- ROADMAP.md (outdated v2.0 roadmap)
- RELEASE_NOTES_v2.1.0.md (superseded by v3.1)

Removed development binaries (360MB+):
- dbbackup (67MB)
- dbbackup_phase2 (67MB)
- dbbackup_phase3 (67MB)
- dbbackup_phase4 (67MB)
- dbbackup_sprint4 (67MB)
- dbbackup_medium (17MB)
- dbbackup_linux_amd64 (47MB)

Updated .gitignore:
- Ignore built binaries in root directory
- Keep bin/ for official releases
- Added IDE and temp file patterns

Result: Cleaner public repository, reduced git size
Kept: Public docs (README, PITR, DOCKER, CLOUD, AZURE, GCS),
      test scripts, build scripts, docker-compose files

.gitignore

@@ -8,3 +8,27 @@ logs/
 *.out
 *.trace
 *.err
+
+# Ignore built binaries in root (keep bin/ directory for releases)
+/dbbackup
+/dbbackup_*
+!dbbackup.png
+
+# Ignore development artifacts
+*.swp
+*.swo
+*~
+.DS_Store
+
+# Ignore IDE files
+.vscode/
+.idea/
+*.iml
+
+# Ignore test coverage
+*.cover
+coverage.html
+
+# Ignore temporary files
+tmp/
+temp/