Align SignalGrid branding to runtime decision-layer positioning

[DanFashauer]

Motivation

• Bring repository branding and documentation into alignment with the product’s current positioning as a runtime decision layer / Zero Trust orchestration platform rather than a generic identity platform.
• Establish a compact, enterprise-grade visual direction (warm charcoal, off-white typography, muted teal accents, minimal/architectural forms) and discourage legacy cyber/shield clichés.
• Provide a single source-of-truth brand system to guide README, demo surfaces, and future visual asset work while preserving MVP/demo claim boundaries.

Description

• Rewrote startup/branding/brand.md to replace the old “Identity Platform” framing with a runtime decision-layer positioning, updated voice/tone, visual direction, and color/type guidance.
• Reworked startup/branding/logo_concepts.md to deprecate shield/badge/cyber concepts and define preferred directions for an abstract S/G monogram and a decision-core mark with construction rules and usage guidance.
• Added docs/BRAND_SYSTEM.md as a new canonical brand system containing positioning, tone, visual principles, color palette, typography guidance, logo constraints, anti-patterns, and repo/website/demo usage rules.
• Lightly updated README.md to include a concise brand and presentation direction section linking to the new docs and brand guide.
• Files changed: startup/branding/brand.md, startup/branding/logo_concepts.md, docs/BRAND_SYSTEM.md (new), and README.md.

Testing

• Ran git diff --check and it passed with no whitespace or diff-check issues.
• Ran npm run lint (ESLint) and it completed successfully with no reported lint failures.

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Codex Task