feat: Add version bump script for consistent versioning

Adds scripts/bump-version.sh to automate version syncing across all
beads components, preventing version mismatches like bd-66.

Features:
- Updates all version files in one command
- Validates semantic versioning format
- Verifies all versions match after update
- Shows git diff preview
- Optional auto-commit with standardized message
- Cross-platform compatible (macOS/Linux)

Files updated by script:
- cmd/bd/version.go
- .claude-plugin/plugin.json
- .claude-plugin/marketplace.json
- integrations/beads-mcp/pyproject.toml
- README.md
- PLUGIN.md

Usage:
  ./scripts/bump-version.sh 0.9.3           # Dry run with diff
  ./scripts/bump-version.sh 0.9.3 --commit  # Auto-commit

Closes bd-67

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

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

README.md

@@ -1120,8 +1120,14 @@ go build -o bd ./cmd/bd
 
 # Run
 ./bd create "Test issue"
+
+# Bump version
+./scripts/bump-version.sh 0.9.3           # Update all versions, show diff
+./scripts/bump-version.sh 0.9.3 --commit  # Update and auto-commit
 ```
 
+See [scripts/README.md](scripts/README.md) for more development scripts.
+
 ## License
 
 MIT