feat: Add bd info --whats-new for agent version awareness (bd-eiz9)

- Add --whats-new flag to bd info command
- Display agent-relevant changes from last 3 versions
- Support both human-readable and JSON output
- Add version changelog data structure with workflow-impacting changes
- Add comprehensive tests for version changes structure
- Update AGENTS.md with whats-new documentation

Helps agents efficiently learn what changed between bd versions without
re-reading full documentation. Addresses weekly major version releases
requiring workflow adaptations.

Closes bd-eiz9

Amp-Thread-ID: https://ampcode.com/threads/T-5fe7e93d-7398-41c5-94bf-e914f2b331dd
Co-authored-by: Amp <amp@ampcode.com>

AGENTS.md

@@ -4,6 +4,23 @@
 
 This is **beads** (command: `bd`), an issue tracker designed for AI-supervised coding workflows. We dogfood our own tool!
 
+## 🆕 What's New?
+
+**New to bd or upgrading?** Run `bd info --whats-new` to see agent-relevant changes from recent versions:
+
+```bash
+bd info --whats-new          # Human-readable output
+bd info --whats-new --json   # Machine-readable output
+```
+
+This shows the last 3 versions with workflow-impacting changes, avoiding the need to re-read all documentation. Examples:
+- New commands and flags that improve agent workflows
+- Breaking changes that require workflow updates
+- Performance improvements and bug fixes
+- Integration features (MCP, Agent Mail, git hooks)
+
+**Why this matters:** bd releases weekly with major versions. This command helps you quickly understand what changed without parsing the full CHANGELOG.
+
 ## Human Setup vs Agent Usage
 
 **IMPORTANT:** If you need to initialize bd, use the `--quiet` flag: