docs: update CHANGELOG for v0.2.4 release

Add 0.2.4 changelog entry covering:
- Priming subsystem (PRIME.md, post-handoff detection, doctor checks)
- gt prime --dry-run, --state, --explain flags
- ZFC improvements (query tmux directly, remove PID detection)
- Cross-level hook visibility fixes
- Rig-level default formulas

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

internal/doctor/priming_check.go

@@ -149,6 +149,20 @@ func (c *PrimingCheck) checkAgentPriming(townRoot, agentDir, agentType string) [
 		}
 	}
 
+	// Check AGENTS.md is minimal (bootstrap pointer, not full context)
+	agentsMdPath := filepath.Join(agentPath, "AGENTS.md")
+	if fileExists(agentsMdPath) {
+		lines := c.countLines(agentsMdPath)
+		if lines > 20 {
+			issues = append(issues, primingIssue{
+				location:    agentDir,
+				issueType:   "large_agents_md",
+				description: fmt.Sprintf("AGENTS.md has %d lines (should be <20 for bootstrap pointer)", lines),
+				fixable:     false, // Full context should come from gt prime templates
+			})
+		}
+	}
+
 	return issues
 }