chore: remove dead migration tooling, stale README steps, and unused assets

[devin-ai-integration]

Summary

Removes dead/stale code that no longer serves any purpose. No logic changes.

Deleted files:

• scripts/create-migration.ts — creates SQL files in a migrations/ directory that no longer exists
• readme-assets/logo-circle.png — only referenced in a commented-out README block
• readme-assets/preview.png — zero references anywhere in repo
• public/graphics/docs/analyze-data-chart.png and static-chart.png — orphaned from a removed docs port (c07895da); the consuming src/app/docs and src/content/docs trees were deleted long ago

package.json script cleanup:

• Removed db:migrations:create, db:migrations:apply (broken — calls nonexistent scripts:apply-migrations), scripts:create-migration, and the empty <<<<<<< Database section header
• Removed redundant scripts:check-all-env alias (was just bun scripts:check-app-env); inlined its 3 callers (test:run, test:watch, test:ui) to use scripts:check-app-env directly

README.md cleanup:

• Title: # E2B Dashboard → # Dashboard
• Updated stack version: Next.js 15 → Next.js 16 (overview + features)
• Updated prereq: Node.js 18+ → Node.js 20.9+ (Next.js 16's actual minimum)
• Removed stale **Documentation**: Integrated MDX documentation support feature line — MDX docs feature was removed
• Removed PostHog and Plain prerequisites (no longer relevant for local setup)
• Removed "Database Setup" step pointing at a non-existent /migrations folder; renumbered subsequent steps
• Removed two non-existent script references (bun run db:types, bun run db:migration)
• Removed commented-out <p align="center">…logo-circle.png…</p> block
• Updated Project Structure section to match actual src/ layout: added configs/, core/, trpc/; removed non-existent server/; refreshed descriptions

Review & Testing Checklist for Human

• Verify no remaining references to create-migration, db:migrations, scripts:check-all-env, MDX docs, Plain, or PostHog in any docs you care about
• Confirm README setup steps still read correctly after renumbering (c → Supabase Storage, d → Start dev server)
• Confirm Next.js 16 / Node 20.9+ versions are the ones you want documented

Notes

• knip reports 0 unused files and 0 unused dependencies after this change (unchanged from main)
• Biome lint + format pass cleanly; pre-existing unit test failures are unchanged
• Separately noted: .github/workflows/sync-infra-specs.yml:76 references stale paths (src/types/* instead of src/core/shared/contracts/*) — left alone since fixing it changes workflow behavior

Link to Devin session: https://app.devin.ai/sessions/bf92d86408ce4a6496ae425d4a9a9ac5

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