Implement JSONL export/import and shift to text-first architecture

This is a fundamental architectural shift from binary SQLite to JSONL as
the source of truth for git workflows.

## New Features

- `bd export --format=jsonl` - Export issues to JSON Lines format
- `bd import` - Import issues from JSONL (create new, update existing)
- `--skip-existing` flag for import to only create new issues

## Architecture Change

**Before:** Binary SQLite database committed to git
**After:** JSONL text files as source of truth, SQLite as ephemeral cache

Benefits:
- Git-friendly text format with clean diffs
- AI-resolvable merge conflicts (append-only is 95% conflict-free)
- Human-readable issue tracking in git
- No binary merge conflicts

## Documentation

- Updated README with JSONL-first workflow and git hooks
- Added TEXT_FORMATS.md analyzing JSONL vs CSV vs binary
- Updated GIT_WORKFLOW.md with historical context
- .gitignore now excludes *.db, includes .beads/*.jsonl

## Implementation Details

- Export sorts issues by ID for consistent diffs
- Import handles both creates and updates atomically
- Proper handling of pointer fields (EstimatedMinutes)
- All tests passing

## Breaking Changes

- Database files (*.db) should now be gitignored
- Use export/import workflow for git collaboration
- Git hooks recommended for automation

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

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

internal/types/types.go

@@ -98,15 +98,16 @@ type Dependency struct {
 type DependencyType string
 
 const (
-	DepBlocks      DependencyType = "blocks"
-	DepRelated     DependencyType = "related"
-	DepParentChild DependencyType = "parent-child"
+	DepBlocks         DependencyType = "blocks"
+	DepRelated        DependencyType = "related"
+	DepParentChild    DependencyType = "parent-child"
+	DepDiscoveredFrom DependencyType = "discovered-from"
 )
 
 // IsValid checks if the dependency type value is valid
 func (d DependencyType) IsValid() bool {
 	switch d {
-	case DepBlocks, DepRelated, DepParentChild:
+	case DepBlocks, DepRelated, DepParentChild, DepDiscoveredFrom:
 		return true
 	}
 	return false