Add Proxmox VE HTTP sample, harden agent-facing docs, scrub stale SPP-version prerequisites

[petrsnd]

Three related cleanups:

1. New sample: samples/http/proxmox-ve-http/ — Proxmox VE platform script (CheckSystem / CheckPassword / ChangePassword) using the ticket-cookie + CSRFPreventionToken auth pattern and the SkipServerCertValidation reserved parameter. Verified against a live appliance in both service-account mode (cross-user rotation) and self-managed mode (a @pve user rotating its own password). Includes a full README documenting both modes, the role/privilege requirements, the @pve-realm-only limitation, and the absence of @pam and API-token support.
2. Agent-facing documentation hardening — every change grounded in a concrete failure mode hit while authoring the new sample: - script-authoring skill: new pre-flight subsection in the http-api recipe (per-Request IgnoreServerCertAuthentication, form bodies via SetFormValue, URL encoding via UrlEncode, grep reserved-parameters.md before declaring custom params).

• safeguard-ps-operations skill: omit -IdentityProvider by default (hardcoding local breaks non-local-IdP users); device-code section rewritten to flag the Out-Null and sync-stdout-buffering traps.
• AGENTS.md: platform display name and operations-needed are now hard pre-conditions before drafting; operations bullet enumerates all four credential families (Password, SshKey, ApiKey, File) plus discovery so credential-intent isn't silently coerced toward passwords.
• Failure-pattern catalog and strategy decision tree updated with the new entries and a forward cite to the new sample.
• tools/TestTool.ps1 and two human-facing guides: -IdentityProvider local examples removed.

3. Repo-wide README cleanup: dropped stale "SPP 6.0 or later" / "7.4 or later" prerequisites from 17 sample READMEs.