docs: correct stale build, forge and layer facts

BUILDING.md never mentioned the floret-kit submodule, so its instructions
could not produce a working build — a plain clone fails to configure. It
also understated the SDK requirement: compileSdk is 37, not the "36+" it
claimed. Both matter now that the Codeberg move is meant to make an
outside clone viable.

The docs map advertised superpowers/specs/ and superpowers/plans/, neither
of which exists any more, while omitting BUILDING.md and RELEASING.md.
Dropped the dead rows and listed what is actually there. Purged the
remaining superpowers references in .planning/ and ARCHITECTURE.md's
dangling "see plan 03" the same way — a pointer to a deleted file is worse
than no pointer.

ARCHITECTURE.md still called OptionCard "the app's only sanctioned
selection-dialog style", which the 2026-07-07 flip to full-screen pickers
reversed, and described no layer for floret-kit despite it being a
compiled-from-source composite build. Its CI note credited Gitea Actions;
contributor CI runs on Codeberg now.

PROJECT.md's Source section described the old world outright ("hosted on
self-hosted Gitea") and now records the split: Codeberg canonical, Gitea
build infrastructure.

.planning/PROJECT.md

@@ -40,5 +40,9 @@ shows a stylized "1" on a slate squircle.
 
 ## Source
 
-Hosted on self-hosted Gitea, released through self-hosted F-Droid repo on
-Hetzner. Same infrastructure as `HouseHoldKeaper`.
+**Codeberg (`jlmakiola/calendula`) is canonical** — git, issues, PRs, tags and
+releases, plus contributor CI. The self-hosted Gitea instance is build
+infrastructure only: it holds the signing key, runs the release pipeline, and
+publishes the self-hosted F-Droid repo on Hetzner. Codeberg push-mirrors `main`
+and tags to Gitea, where a bumped `versionName` triggers the release. Also
+published to the official F-Droid repo. See `docs/RELEASING.md`.