fix(seo): I8 — drop the phantom .claude/audits/external/ precondition

STEP 0 told the user to run `mkdir -p .claude/audits/external` themselves
before handing over an external report. Three things wrong with that:

- The skill runs dozens of bash commands but outsourced this one to a human.
- The timing was impossible: to "drop the export in" that directory the user
  needed it to already exist, so the instruction arrived after the moment it
  would have been useful.
- The directory is not needed at all. `:218` already reads "File path given
  → Read it" — any path works — and nothing in skills/ or agents/ ever
  writes to that path. Grep confirms it is referenced by exactly these two
  lines and known to nothing else: a convention the skill invented, asked
  the user to create, and never used.

Fix removes the precondition instead of automating it: give a path from
anywhere, the tidy location stays a suggestion.

Verified: make test 35 GREEN / 0 RED.

skills/seo/SKILL.md

@@ -204,9 +204,9 @@ Ask ONCE before dispatching the agents:
 ```
 RAPPORT EXTERNE (optionnel) — un autre regard sur le site :
 
-  1. Fichier  — déposez l'export (PDF/MD/TXT) dans
-     `.claude/audits/external/` (ex. `sorank-YYYY-MM-DD.pdf`),
-     donnez le nom du fichier. (`mkdir -p .claude/audits/external`)
+  1. Fichier  — donnez le chemin de l'export (PDF/MD/TXT), où qu'il soit
+     (ex. `~/Téléchargements/sorank-2026-07-16.pdf`). Rangement conseillé
+     mais optionnel : `.claude/audits/external/`.
   2. Collé    — collez ici le contenu du PDF ou le "prompt pour IA"
      que l'outil suggère.
   3. Ignorer  — continuer sans. Le rapport final recommandera