fix(mail): use slash-based identity format for addresses

Changes mail identity format from hyphenated (gastown-Toast) to
slash-based (gastown/polecats/Toast) to:
- Match directory structure (REST-like)
- Distinguish from hyphenated bead IDs (gt-xyz) and molecule names

Updated:
- addressToIdentity(): preserve slashes instead of replacing with dashes
- identityToAddress(): simplified, identity == address now
- detectSender(): include /polecats/ in polecat addresses
- Tests updated for new format

Closes gt-cxtu: shared beads architecture verified working.

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

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

internal/mail/mailbox.go

@@ -20,7 +20,7 @@ var (
 
 // Mailbox manages messages for an identity via beads.
 type Mailbox struct {
-	identity string // beads identity (e.g., "gastown-Toast")
+	identity string // beads identity (e.g., "gastown/polecats/Toast")
 	workDir  string // directory to run bd commands in
 	beadsDir string // explicit .beads directory path (set via BEADS_DIR)
 	path     string // for legacy JSONL mode (crew workers)