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>

examples/bd-example-extension-go/README.md

@@ -182,7 +182,7 @@ jsonlPath := beads.FindJSONLPath(dbPath)
 
 ```go
 // Open same database for extension tables
-db, err := sql.Open("sqlite", dbPath)
+db, err := sql.Open("sqlite3", dbPath)
 
 // Initialize extension schema
 _, err = db.Exec(Schema)

examples/bd-example-extension-go/main.go

@@ -30,7 +30,7 @@ func main() {
 	// Open bd storage + extension database
 	store, _ := beads.NewSQLiteStorage(*dbPath)
 	defer store.Close()
-	db, _ := sql.Open("sqlite", *dbPath)
+	db, _ := sql.Open("sqlite3", *dbPath)
 	defer db.Close()
 	db.Exec("PRAGMA journal_mode=WAL")
 	db.Exec("PRAGMA busy_timeout=5000")