fix(doctor): add sqlite3 availability check (#575)

- Add sqlite3 to README.md prerequisites section
- Add gt doctor check that warns if sqlite3 CLI is not found
- Documents that sqlite3 is required for convoy database queries

Fixes #534

Co-authored-by: Claude Opus 4.5 <noreply@anthropic.com>

internal/cmd/doctor.go

@@ -118,6 +118,7 @@ func runDoctor(cmd *cobra.Command, args []string) error {
 
 	// Register built-in checks
 	d.Register(doctor.NewStaleBinaryCheck())
+	d.Register(doctor.NewSqlite3Check())
 	d.Register(doctor.NewTownGitCheck())
 	d.Register(doctor.NewTownRootBranchCheck())
 	d.Register(doctor.NewPreCheckoutHookCheck())