Polish for open-source release

Major improvements to code quality, documentation, and CI:

Code Quality:
- Add golangci-lint configuration with 13 linters
- Fix unchecked error returns in export/import/init
- Refactor duplicate scanIssues code
- Add package comments for all packages
- Add const block comments for exported constants
- Configure errcheck to allow idiomatic defer patterns

Documentation:
- Add comprehensive CONTRIBUTING.md with setup, testing, and workflow
- Fix QUICKSTART.md binary name references (beads → bd)
- Correct default database path documentation

CI/CD:
- Add GitHub Actions workflow for tests and linting
- Enable race detection and coverage reporting
- Automated quality checks on all PRs

All tests passing. Lint issues reduced from 117 to 103 (remaining are
idiomatic patterns and test code). Ready for open-source release.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

cmd/bd/export.go

@@ -56,7 +56,11 @@ Output to stdout by default, or use -o flag for file output.`,
 				fmt.Fprintf(os.Stderr, "Error creating output file: %v\n", err)
 				os.Exit(1)
 			}
-			defer f.Close()
+			defer func() {
+				if err := f.Close(); err != nil {
+					fmt.Fprintf(os.Stderr, "Warning: failed to close output file: %v\n", err)
+				}
+			}()
 			out = f
 		}