docs: add task breakdown (branches, streams, commit-sized tasks)

[gitjehan]

Summary

• Adds docs/TASKS.md, the companion doc to PLAN.md/IMPLEMENTATION.md: a commit-sized task breakdown organized into parallel work streams (A–H) with a branch/PR workflow and a parallelism map for the team.
• Covers backend proxy, app bring-up, privacy strip, step engine, senior UI, prompt work, dashboard/landing, and demo prep, each with per-commit task lists and done criteria.

Notes

• This branch also accidentally includes a committed .DS_Store; worth dropping (and adding to .gitignore) in a follow-up commit before merge.