Enhance bd onboard with AI planning docs management guidance

Add comprehensive guidance for managing AI-generated planning documents
(Claude slop) to the bd onboard command. Addresses GitHub issue #196.

Changes:
- Add Managing AI-Generated Planning Documents section to onboard output
- Recommend using history/ directory for ephemeral planning files
- List common planning doc types (PLAN.md, IMPLEMENTATION.md, etc.)
- Provide .gitignore example and benefits explanation
- Add to Important Rules: Store AI planning docs in history/
- Update AGENTS.md with the same guidance to demonstrate the pattern
- Add comprehensive tests for onboard command

This uses inverted control: bd outputs guidance, AI agents intelligently
integrate it into their project's documentation (AGENTS.md, CLAUDE.md,
.cursorrules, etc.), respecting existing conventions.

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

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

AGENTS.md

@@ -950,15 +950,45 @@ Add to MCP config (e.g., `~/.config/claude/config.json`):
 
 Then use `mcp__beads__*` functions instead of CLI commands.
 
+### Managing AI-Generated Planning Documents
+
+AI assistants often create planning and design documents during development:
+- PLAN.md, IMPLEMENTATION.md, ARCHITECTURE.md
+- DESIGN.md, CODEBASE_SUMMARY.md, INTEGRATION_PLAN.md
+- TESTING_GUIDE.md, TECHNICAL_DESIGN.md, and similar files
+
+**Best Practice: Use a dedicated directory for these ephemeral files**
+
+**Recommended approach:**
+- Create a `history/` directory in the project root
+- Store ALL AI-generated planning/design docs in `history/`
+- Keep the repository root clean and focused on permanent project files
+- Only access `history/` when explicitly asked to review past planning
+
+**Example .gitignore entry (optional):**
+```
+# AI planning documents (ephemeral)
+history/
+```
+
+**Benefits:**
+- ✅ Clean repository root
+- ✅ Clear separation between ephemeral and permanent documentation
+- ✅ Easy to exclude from version control if desired
+- ✅ Preserves planning history for archaeological research
+- ✅ Reduces noise when browsing the project
+
 ### Important Rules
 
 - ✅ Use bd for ALL task tracking
 - ✅ Always use `--json` flag for programmatic use
 - ✅ Link discovered work with `discovered-from` dependencies
 - ✅ Check `bd ready` before asking "what should I work on?"
+- ✅ Store AI planning docs in `history/` directory
 - ❌ Do NOT create markdown TODO lists
 - ❌ Do NOT use external issue trackers
 - ❌ Do NOT duplicate tracking systems
+- ❌ Do NOT clutter repo root with planning documents
 
 For more details, see README.md and QUICKSTART.md.
 <!-- /bd onboard section -->