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.sum

@@ -1,10 +1,6 @@
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