refactor(development): move gastown patches to separate files

Replace inline postPatch substituteInPlace calls with proper unified
diff patch files, following the pattern established by beads.

This improves maintainability:
- Each patch is in its own file with clear naming
- Patches use proper unified diff format
- Easier to review, update, and track individual fixes
- Default.nix is cleaner (237 lines of substituteInPlace -> 15 lines)

Patches included:
- gastown-fix-validate-recipient.patch
- gastown-fix-agent-bead-address-title.patch
- gastown-fix-agent-bead-rig-prefix.patch
- gastown-fix-role-home-paths.patch
- gastown-fix-town-root-detection.patch
- gastown-fix-copydir-symlinks.patch
- gastown-statusline-optimization.patch

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

home/roles/development/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
+ 		}
+ 	}