refactor: split gt sling into gt hook + gt handoff <bead> (gt-z4bw)

- Add gt hook <bead>: durability primitive, attaches work to hook
- Update gt handoff: accept optional bead arg (detects bead vs role)
- Deprecate gt sling: shows warning, points to new commands
- Update doctor fix hint to reference new commands

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

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

internal/doctor/hook_check.go

@@ -388,7 +388,7 @@ func (c *OrphanedAttachmentsCheck) Run(ctx *CheckContext) *CheckResult {
 		Status:  StatusWarning,
 		Message: fmt.Sprintf("Found %d orphaned handoff bead(s)", len(c.orphans)),
 		Details: details,
-		FixHint: "Re-sling molecule to active agent with 'gt sling', or close with 'bd close <id>'",
+		FixHint: "Re-hook molecule with 'gt hook <id>' then 'gt handoff', or close with 'bd close <id>'",
 	}
 }