feat(cli): add bd pin/unpin commands (bd-iea)

Add pin.go and unpin.go to cmd/bd/ for managing pinned issues.
- bd pin <id> sets Pinned=true on an issue
- bd unpin <id> sets Pinned=false on an issue

Also adds Pinned field support to RPC UpdateArgs for daemon mode.

🤝 Slit

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

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

internal/rpc/server_issues_epics.go

@@ -97,6 +97,10 @@ func updatesFromArgs(a UpdateArgs) map[string]interface{} {
 	if a.SupersededBy != nil {
 		u["superseded_by"] = *a.SupersededBy
 	}
+	// Pinned field (bd-iea)
+	if a.Pinned != nil {
+		u["pinned"] = *a.Pinned
+	}
 	return u
 }