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>

internal/types/types.go

@@ -1,3 +1,4 @@
+// Package types defines core data structures for the bd issue tracker.
 package types
 
 import (
@@ -49,6 +50,7 @@ func (i *Issue) Validate() error {
 // Status represents the current state of an issue
 type Status string
 
+// Issue status constants
 const (
 	StatusOpen       Status = "open"
 	StatusInProgress Status = "in_progress"
@@ -68,6 +70,7 @@ func (s Status) IsValid() bool {
 // IssueType categorizes the kind of work
 type IssueType string
 
+// Issue type constants
 const (
 	TypeBug     IssueType = "bug"
 	TypeFeature IssueType = "feature"
@@ -97,6 +100,7 @@ type Dependency struct {
 // DependencyType categorizes the relationship
 type DependencyType string
 
+// Dependency type constants
 const (
 	DepBlocks         DependencyType = "blocks"
 	DepRelated        DependencyType = "related"
@@ -134,6 +138,7 @@ type Event struct {
 // EventType categorizes audit trail events
 type EventType string
 
+// Event type constants for audit trail
 const (
 	EventCreated           EventType = "created"
 	EventUpdated           EventType = "updated"