Better enable go extensions (#14)

* deps: run go mod tidy

* beads: Add public Go API for bd extensions

Implements a minimal public API to enable Go-based extensions without
exposing internal packages:

**New beads.go package:**
- Exports essential types: Issue, Status, IssueType, WorkFilter
- Provides status and issue type constants
- Exposes NewSQLiteStorage() as main entry point for extensions
- Includes comprehensive package documentation

**Updated EXTENDING.md:**
- Replaced internal package imports with public beads package
- Updated function calls to use new public API
- Changed sqlite.New() to beads.NewSQLiteStorage()
- Updated GetReady() to GetReadyWork() with WorkFilter

This enables clean Go-based orchestration extensions while maintaining
API stability and hiding internal implementation details.

* beads: Refine Go extensions API and documentation

Updates to the public Go API implementation following initial commit:

- Enhanced beads.go with refined extension interface
- Updated EXTENDING.md with clearer documentation
- Modified cmd/bd/main.go to support extension loading

Continues work on enabling Go-based bd extensions.

* Fix EXTENDING.md to use beads.WorkFilter instead of types.WorkFilter

The public API exports WorkFilter as beads.WorkFilter, not types.WorkFilter.
This fixes the code example to match the imports shown.

---------

Co-authored-by: Steve Yegge <steve.yegge@gmail.com>

go.mod

@@ -3,23 +3,15 @@ module github.com/steveyegge/beads
 go 1.21
 
 require (
-	github.com/fatih/color v1.18.0 // indirect
-	github.com/fsnotify/fsnotify v1.9.0 // indirect
-	github.com/go-viper/mapstructure/v2 v2.4.0 // indirect
+	github.com/fatih/color v1.18.0
+	github.com/mattn/go-sqlite3 v1.14.32
+	github.com/spf13/cobra v1.10.1
+)
+
+require (
 	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
-	github.com/mattn/go-sqlite3 v1.14.32 // indirect
-	github.com/pelletier/go-toml/v2 v2.2.4 // indirect
-	github.com/sagikazarmark/locafero v0.11.0 // indirect
-	github.com/sourcegraph/conc v0.3.1-0.20240121214520-5f936abd7ae8 // indirect
-	github.com/spf13/afero v1.15.0 // indirect
-	github.com/spf13/cast v1.10.0 // indirect
-	github.com/spf13/cobra v1.10.1 // indirect
 	github.com/spf13/pflag v1.0.10 // indirect
-	github.com/spf13/viper v1.21.0 // indirect
-	github.com/subosito/gotenv v1.6.0 // indirect
-	go.yaml.in/yaml/v3 v3.0.4 // indirect
 	golang.org/x/sys v0.29.0 // indirect
-	golang.org/x/text v0.28.0 // indirect
 )