Add flags-sdk skill synced from vercel/flags

[vincent-derks]

Adds Vercel Flags coverage as the 12th upstream-synced skill. Before this, flags.ts edits and flags/@flags-sdk/* installs injected nothing, and vercel flags commands only got CLI syntax via vercel-cli — no SDK integration guidance.

Changes

• skills/flags-sdk/ — upstream content vendored from vercel/flags (SKILL.md + 4 references), plus overlay.yaml with injection metadata (path/import/bash patterns, prompt signals, summary fallback since the body slightly exceeds the 18KB budget)
• vercel.md — Vercel Flags product node in the platform tree; feature-flag routing moved from Edge Config to Vercel Flags (decision matrix, Toolbar link, SaaS workflow)
• session-start-profiler — flags.ts file marker and flags/@flags-sdk/vercel package markers
• generate-catalog.ts — replaced stale @vercel/flags probe with current package names, added a vercel flags bash probe
• Docs/counts, regenerated manifest and catalog
• Repo-wide: replaced the docs sitemap URL in 17 skills — vercel.com/sitemap/docs.xml soft-404s; vercel.com/sitemap.xml is the working one (injected into agent context by buildDocsBlock)
• Repo-wide: Edge Config → Global Config rename (July 2026) — prose, docs URLs, code examples, and knowledge-update entry use the new name; legacy @vercel/edge-config patterns are kept as input matchers, and a new recommended validate rule nudges the drop-in @vercel/global-config migration. Also fixed a bogus POST /v1/edge-config/purge reference in the CDN cache table (correct mechanism is vercel cache purge --type cdn)

I tested the plugin locally and it loads the skill correctly:

Verified

• bun test — 974 pass
• bun run validate — passes (previously failed on catalog staleness)
• Explain CLI: flags.ts, src/lib/flags.ts, vercel flags create, and npm install flags @flags-sdk/vercel all inject flags-sdk
• Prompt scoring: "add a feature flag" and "a/b test" select flags-sdk; "cli flag" phrasing is suppressed via noneOf