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>

README.md

@@ -262,7 +262,7 @@ This pattern has proven invaluable for maintaining database hygiene and preventi
 
 **1. File/update issues for remaining work**
 - Agents should proactively create issues for discovered bugs, TODOs, and follow-up tasks
-- Close completed issues and update status for in-progress work
+- Close completed issues and update status for in_progress work
 
 **2. Run quality gates (if applicable)**
 - Tests, linters, builds - only if code changes were made