docs: add implementation plan for the Proton Drive rewrite (M0-M4)

Fork-only. Not for upstream.

Co-Authored-By: Claude Fable 5.2 <noreply@anthropic.com>

docs/TODO.md

@@ -6,12 +6,6 @@ Small, specific, independently actionable. Remove only when actually done.
 
 ## Now
 
-- [ ] Install a Go toolchain on the development host (go.mod needs 1.26; CI
-  builds with 1.27.x). Until then every `go` command has to run in the
-  `golang:1.26` container - see `docs/PERMISSIONS.md`.
-- [ ] Install `golangci-lint` v2 so `golangci-lint run` can be verified and
-  moved into the Allow tier.
-
 ## Next
 
 - [ ] Propose a CI check for this fork: `AGENTS.md`, `CLAUDE.md`, `GEMINI.md`