[codex] Fix immersive agent visual stage

[rogerchappel]

Summary

• Render immersive Agent Mode as a top-level fixed overlay instead of inside the composer agentPanel slot.
• Keep only compact Agent Mode inside the message composer so the message text does not remain visible in immersive mode.
• Strengthen Neural, Hologram, and Command visual layers and add baseline motion so low live audio levels still feel reactive.
• Keep visual style identity tied to the selected accent rather than replacing it with state colors.

Why

Post-merge testing showed immersive mode was still constrained to the message box and the new styles were too faint/static, especially in light mode. The composer slot appears to bound fixed positioning, so immersive needs to be rendered outside that slot.

Verification

• pnpm typecheck
• pnpm test src/lib/agent-visual-settings.test.ts 'src/app/api/companies/[id]/route.test.ts'
• pnpm lint:check (existing warnings only)
• git diff --check origin/main...HEAD
• agent visual immersive smoke check
• push hook: pnpm typecheck and pnpm build