chore: Drop JSON formula support, TOML only (gt-spt8v)

- Delete all .formula.json files (TOML versions exist)
- Update sling.go comment to say TOML only

Note: Doc updates were prepared but those docs were deleted
upstream in the recent refactoring.

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

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

internal/cmd/sling.go

@@ -469,7 +469,7 @@ func resolveSelfTarget() (agentID string, pane string, hookRoot string, err erro
 }
 
 // verifyFormulaExists checks that the formula exists using bd formula show.
-// Formulas can be formula files (.formula.json/.formula.toml).
+// Formulas are TOML files (.formula.toml).
 func verifyFormulaExists(formulaName string) error {
 	// Try bd formula show (handles all formula file formats)
 	cmd := exec.Command("bd", "formula", "show", formulaName)

internal/templates/roles/deacon.md.tmpl

@@ -162,6 +162,20 @@ This is the opposite of polecat work, which is persistent and auditable.
 | Witness | `gt-<rig>-witness` |
 | Crew | `gt-<rig>-<name>` |
 
+## Inbox Hygiene
+
+**CRITICAL**: Always delete messages after handling them. Messages accumulate if not cleared.
+
+```bash
+gt mail inbox                    # Check inbox
+gt mail read <id>                # Read message
+# ... handle the message ...
+gt mail delete <id>              # ALWAYS delete after handling
+```
+
+**Handoff messages** (`🤝 HANDOFF:`) are context notes from your previous session.
+Read them for situational awareness, then delete immediately.
+
 ## Lifecycle Request Handling
 
 When you receive lifecycle mail:
@@ -183,8 +197,8 @@ gt mail read <id>
 gt mayor stop
 gt mayor start
 
-# Acknowledge
-gt mail ack <id>
+# Delete the message
+gt mail delete <id>
 ```
 
 ## Timer Callbacks