Add beads and gastown to CI cache, consolidate CI workflow

- Add packages/beads and packages/gastown with shared definitions
- Expose custom-beads and custom-gastown in flake packages output
- Consolidate CI from matrix (8 parallel jobs) to single job with loop
- Saves ~12 minutes of redundant nix-setup time per run
- Uses ::group:: for collapsible log sections per package

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

packages/gastown/gastown-fix-validate-recipient.patch

@@ -0,0 +1,13 @@
+diff --git a/internal/mail/router.go b/internal/mail/router.go
+index b864c069..4b6a045b 100644
+--- a/internal/mail/router.go
++++ b/internal/mail/router.go
+@@ -646,7 +646,7 @@ func (r *Router) validateRecipient(identity string) error {
+ 	}
+ 
+ 	for _, agent := range agents {
+-		if agentBeadToAddress(agent) == identity {
++		if AddressToIdentity(agentBeadToAddress(agent)) == AddressToIdentity(identity) {
+ 			return nil // Found matching agent
+ 		}
+ 	}