v2: production readiness implementation

[harshmathurx]

Summary

• removes the review-only production readiness plan from the repo
• makes launch copy/README honest about local-first beta and remote sync status
• fixes lint issues and adds ignores for generated static export output
• strengthens local persistence semantics and tests
• adds markdown vault export as ZIP
• gates remote storage behind NEXT_PUBLIC_ENABLE_REMOTE_STORAGE=false by default
• adds GitHub remote storage foundation, encrypted browser token storage, sync queue wiring, and conflict surfacing tests

Validation

• ./node_modules/.bin/tsc --noEmit
• ./node_modules/.bin/vitest run: 5 files, 19 tests passed
• NODE_OPTIONS=--max-old-space-size=4096 ./node_modules/.bin/eslint .
• ./node_modules/.bin/next build

Notes

• Remote storage remains disabled by default for the static build.
• GitHub beta path currently uses a user-provided personal access token; OAuth API routes were removed because this app is configured for static export.
• Dropbox remains deferred behind the provider surface.