fix: critical issues in wisp hook system

Code review fixes:

1. CRITICAL: Move polecat check to start of runSling
   - Previously wrote wisp THEN failed, leaving orphan
   - Now fails fast before any file operations

2. CRITICAL: Sanitize slashes in agent IDs for filenames
   - Agent IDs like 'gastown/crew/joe' were creating subdirs
   - Now converts '/' to '--' for safe filenames
   - Added sanitizeAgentID/unsanitizeAgentID helpers

3. MODERATE: Use git root instead of WorkDir in prime.go
   - Hooks are written to clone root, not cwd
   - Added getGitRoot() helper for consistency

4. MODERATE: Fix silent error swallowing
   - Now logs non-ErrNoHook errors when reading hooks
   - Warns if bead doesn't exist before burning hook
   - Preserves hook if bead is missing for debugging

internal/wisp/io.go

@@ -136,6 +136,7 @@ func HasHook(root, agent string) bool {
 }
 
 // ListHooks returns a list of agents with active hooks.
+// Agent IDs are returned in their original form (with slashes).
 func ListHooks(root string) ([]string, error) {
 	dir := filepath.Join(root, WispDir)
 	entries, err := os.ReadDir(dir)
@@ -152,7 +153,8 @@ func ListHooks(root string) ([]string, error) {
 		if len(name) > len(HookPrefix)+len(HookSuffix) &&
 			name[:len(HookPrefix)] == HookPrefix &&
 			name[len(name)-len(HookSuffix):] == HookSuffix {
-			agent := name[len(HookPrefix) : len(name)-len(HookSuffix)]
+			sanitized := name[len(HookPrefix) : len(name)-len(HookSuffix)]
+			agent := unsanitizeAgentID(sanitized)
 			agents = append(agents, agent)
 		}
 	}