fix: gt sling now hooks + runs (no restart), update docs

Corrected sling behavior:
- gt hook: attach only (durability primitive)
- gt sling: attach + inject run prompt (preserve context)
- gt handoff: attach + restart (fresh context via GUPP)

Sling can now target other agents: gt sling <bead> <target>

Updated docs/sling-design.md with full command menagerie.

🤖 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-hook molecule with 'gt hook <id>' then 'gt handoff', or close with 'bd close <id>'",
+		FixHint: "Reassign with 'gt sling <id> <agent>', or close with 'bd close <id>'",
 	}
 }