Rebrand Phase 6: Update root docs, configs, and cspell

[JustAGhosT]

Context

NexaMesh rebrand — root-level documentation and tooling config.

Files to Update

• README.md — all PhoenixRooivalk references
• CLAUDE.md — project overview
• CONTRIBUTING.md — project name, git clone URL
• SECURITY.md — project references
• RESPONSIBLE_USE.md — mission statement
• cspell.json + config/cspell.json — add "NexaMesh" to words, remove/keep "PhoenixRooivalk"
• cliff.toml — changelog header
• apps/marketing/src/contexts/CartContext.tsx — CART_STORAGE_KEY

Verification

• pnpm lint passes
• pnpm format:check passes
• Spell check passes

[blocksorg]

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