docs: drop the roadmap and state snapshot, superseded by the tracker

ROADMAP.md and STATE.md predate the issue tracker. They were how planning
worked before issues and milestones existed, and keeping them means
maintaining a second copy of that information by hand — which is why
STATE.md still described release/v2.8.0 as the next release, eight minors
ago.

Issues and milestones say the same thing and can't drift: a milestone maps
to its release/vX.Y.Z branch, which is now also how a contributor learns
where to target a PR. Both files stay in git history if a past decision
needs looking up.

Repointed the six references at the tracker: the README's docs list, two
rows in the docs map, PROJECT.md's Current Milestone, and REQUIREMENTS.md's
idea-backlog pointer. ARCHITECTURE.md's "see the roadmap's idea backlog"
now cites the crash reporter's web-issue path instead — a worked example of
the no-network rule beats a pointer to a deleted backlog.

.planning/PROJECT.md

@@ -17,9 +17,10 @@ re-inventing the calendar sync stack — leave that to DAVx5 and the system.
 ## Current Milestone
 
 Milestones 1 (read, v1.0) and 2 (write support, v1.1–v2.0.0 incl. reminder
-delivery) are **complete** — v2.0.0 shipped 2026-06-11. Next is v3.0
-(power-user features) plus an undecided "Locations & People" idea backlog;
-see `ROADMAP.md`.
+delivery) are **complete** — v2.0.0 shipped 2026-06-11. Everything since is
+tracked as issues and milestones on Codeberg:
+<https://codeberg.org/jlmakiola/calendula/milestones>. A milestone maps to its
+`release/vX.Y.Z` branch.
 
 ## Stack