docs(flagship): update API envelope docs and add A/B/n rollout pattern

[akshitsinha]

Updates Flagship Cloudflare skill references for current control-plane behavior and adds guidance for multi-variant A/B/n experiments.

Changes:

• Document current Cloudflare v4 response envelope for management endpoints:
  • payloads under result
  • pagination cursor under result_info.cursor
  • errors under errors[].message
• Remove unsupported legacy response-field usage from examples.
• Update REST examples to use jq '.result'.
• Clarify management API response casing:
  • snake_case fields such as created_at, updated_at, updated_by, default_variation, serve_variation, and changelog flag_key
• Clarify /evaluate response behavior:
  • not wrapped in management envelope
  • OpenFeature-style camelCase (flagKey, variant, reason)
• Add response-shape examples for apps, flags, and changelog entries.
• Add gotchas for envelope access and casing differences between management responses and evaluation responses.
• Add A/B/n multi-variant testing pattern:
  • one rule per variant
  • cumulative rollout thresholds
  • example 30% / 40% / 30% split across variants A/B/C
  • guidance to use conditions: [] and set final threshold to 100

Validation:

• Reviewed branch diff against origin/main.
• Verified no unsupported .data, nextCursor, or single .error references remain in Flagship docs.