fix(docs): use consistent in_progress status naming (GH#444)

Changed all user-facing documentation and help text to use `in_progress`
(underscore) instead of `in-progress` (hyphen) to match the canonical
status value.

Files updated:
- cmd/bd/ready.go - Short description
- cmd/bd/status.go - Long description
- commands/stats.md - Action suggestion
- README.md - Workflow description

Note: CSS class names, HTML IDs, and GitHub label mappings intentionally
kept with hyphens as they follow different conventions.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

commands/stats.md

@@ -13,5 +13,5 @@ Use the beads MCP `stats` tool to retrieve project metrics and present them clea
 
 Optionally suggest actions based on the stats:
 - High number of blocked issues? Run `/bd-blocked` to investigate
-- No in-progress work? Run `/bd-ready` to find tasks
+- No in_progress work? Run `/bd-ready` to find tasks
 - Many open issues? Consider prioritizing with `/bd-update`