Add siade-probe-provider-charset skill

[Samuelfaure]

Summary

• Generalizes the ad-hoc methodology used on PR Reject digits and brackets in Civility::ValidatePrenoms #341 (determining which characters MEN/MESRI/CNOUS/GIP-MDS actually accept in prenoms) into a reusable project skill under siade/.claude/skills/siade-probe-provider-charset/.
• Documents the full methodology: filter to providers that actually forward the field under test (skip ones that only use it for local identity matching, e.g. ANTS), pull a valid baseline from config/pings.yml/specs, build an isolated one-character-at-a-time variation matrix (never combine two suspect characters in one string), run against production via the provider's MakeRequest interactor directly, and read HTTP status to tell "format accepted" (functional 404) from "format rejected" (genuine 400/422) rather than assuming.
• Cross-references the existing siade-ping-provider skill for the exact per-provider call shape (token/organizer_params/recipient).

Test plan

• N/A — documentation-only skill file, no application code changed.