chore(tooling): add structural lint guardrails

[Teeldinho]

Issue

Structural architecture conventions were documented but not fully enforced by blocking checks. Runtime/test warning paths also produced noisy output, making failures harder to spot.

Solution

• Add structural lint guardrails for FSD public API boundaries, static config purity, render-only UI files, static inline styles, and deprecated shared utils imports.
• Move ast-grep config to root sgconfig.yml and make structural violations fail lint:structure.
• Add filesystem checks for flat FSD segment folders and required slice public APIs.
• Update Convex AI guidance and local agent skill assets.
• Stabilize runtime warning paths for GLTF preloading, Three.js tests, haptics, settings, and interaction prompts.

Screenshots

N/A — tooling, guidance, and runtime warning-path changes only.

Testing

Added structural-lint rule fixtures covering valid and invalid examples for each new ast-grep rule.

Added regression coverage for warning-path behavior around GLTF preloading, Three.js test setup, haptics, and interaction prompts.

Validated the full local quality gate before pushing.