Fix SQLite driver name mismatch causing "unknown driver" errors (#252)

* fix: Use correct SQLite driver name 'sqlite3' instead of 'sqlite'

The ncruces/go-sqlite3 driver registers as 'sqlite3', but doctor.go
and example code were using 'sqlite', causing 'unknown driver' errors.

This fix corrects all sql.Open() calls to use the proper driver name:
- cmd/bd/doctor.go: 6 instances fixed
- docs/EXTENDING.md: 2 documentation examples updated
- examples/bd-example-extension-go/: Fixed example code and README

Fixes #230

Amp-Thread-ID: https://ampcode.com/threads/T-1e8c5473-cb79-4457-be07-4517bfdb73f4
Co-authored-by: Amp <amp@ampcode.com>

* Revert CGO_ENABLED back to 0 for pure-Go SQLite driver

The ncruces/go-sqlite3 driver is pure-Go and doesn't require CGO.
The previous change to CGO_ENABLED=1 in commit f9771cd was an
attempted fix for #230, but the real issue was the driver name
mismatch ('sqlite' vs 'sqlite3'), which is now fixed.

Benefits of CGO_ENABLED=0:
- Simpler cross-compilation (no C toolchain required)
- Smaller binaries
- Faster builds
- Matches the intended design of the pure-Go driver

---------

Co-authored-by: Amp <amp@ampcode.com>

cmd/bd/doctor.go

@@ -314,7 +314,7 @@ func checkIDFormat(path string) doctorCheck {
 	}
 
 	// Open database
-	db, err := sql.Open("sqlite", dbPath+"?mode=ro")
+	db, err := sql.Open("sqlite3", dbPath+"?mode=ro")
 	if err != nil {
 		return doctorCheck{
 			Name:    "Issue IDs",
@@ -400,7 +400,7 @@ func checkCLIVersion() doctorCheck {
 }
 
 func getDatabaseVersionFromPath(dbPath string) string {
-	db, err := sql.Open("sqlite", dbPath+"?mode=ro")
+	db, err := sql.Open("sqlite3", dbPath+"?mode=ro")
 	if err != nil {
 		return "unknown"
 	}
@@ -785,7 +785,7 @@ func checkDatabaseJSONLSync(path string) doctorCheck {
 	jsonlCount, jsonlPrefixes, jsonlErr := countJSONLIssues(jsonlPath)
 
 	// Single database open for all queries (instead of 3 separate opens)
-	db, err := sql.Open("sqlite", dbPath)
+	db, err := sql.Open("sqlite3", dbPath)
 	if err != nil {
 		// Database can't be opened. If JSONL has issues, suggest recovery.
 		if jsonlErr == nil && jsonlCount > 0 {
@@ -990,7 +990,7 @@ func checkPermissions(path string) doctorCheck {
 	dbPath := filepath.Join(beadsDir, beads.CanonicalDatabaseName)
 	if _, err := os.Stat(dbPath); err == nil {
 		// Try to open database
-		db, err := sql.Open("sqlite", dbPath)
+		db, err := sql.Open("sqlite3", dbPath)
 		if err != nil {
 			return doctorCheck{
 				Name:    "Permissions",
@@ -1038,7 +1038,7 @@ func checkDependencyCycles(path string) doctorCheck {
 	}
 
 	// Open database to check for cycles
-	db, err := sql.Open("sqlite", dbPath)
+	db, err := sql.Open("sqlite3", dbPath)
 	if err != nil {
 		return doctorCheck{
 			Name:    "Dependency Cycles",