Add UnderlyingDB() method for extension database access

Implements database platform layer for extensions like VC to create
their own tables in the same SQLite database.

Changes:
- Add UnderlyingDB() *sql.DB to Storage interface
- Implement in SQLiteStorage to expose underlying connection
- Add comprehensive test suite (5 tests, -race clean)
- Tests cover: basic access, extension tables, concurrency,
  lifecycle safety, and transaction behavior

This allows VC to host its executor_instances and other tables
alongside beads core tables with proper FK enforcement.

Related issues: bd-57, bd-64, bd-65, bd-66

Amp-Thread-ID: https://ampcode.com/threads/T-a6715beb-fe92-4dee-b931-3c9327124875
Co-authored-by: Amp <amp@ampcode.com>

go.mod

@@ -2,17 +2,22 @@ module github.com/steveyegge/beads
 
 go 1.24.0
 
+toolchain go1.24.2
+
 require (
 	github.com/anthropics/anthropic-sdk-go v1.14.0
 	github.com/fatih/color v1.18.0
+	github.com/google/uuid v1.6.0
 	github.com/spf13/cobra v1.10.1
+	golang.org/x/mod v0.29.0
+	golang.org/x/sys v0.36.0
+	gopkg.in/natefinch/lumberjack.v2 v2.2.1
 	modernc.org/sqlite v1.38.2
 	rsc.io/script v0.0.2
 )
 
 require (
 	github.com/dustin/go-humanize v1.0.1 // indirect
-	github.com/google/uuid v1.6.0 // indirect
 	github.com/inconshreveable/mousetrap v1.1.0 // indirect
 	github.com/mattn/go-colorable v0.1.13 // indirect
 	github.com/mattn/go-isatty v0.0.20 // indirect
@@ -24,10 +29,7 @@ require (
 	github.com/tidwall/pretty v1.2.1 // indirect
 	github.com/tidwall/sjson v1.2.5 // indirect
 	golang.org/x/exp v0.0.0-20250620022241-b7579e27df2b // indirect
-	golang.org/x/mod v0.29.0 // indirect
-	golang.org/x/sys v0.36.0 // indirect
 	golang.org/x/tools v0.37.0 // indirect
-	gopkg.in/natefinch/lumberjack.v2 v2.2.1 // indirect
 	modernc.org/libc v1.66.3 // indirect
 	modernc.org/mathutil v1.7.1 // indirect
 	modernc.org/memory v1.11.0 // indirect