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/STATE.md

@@ -25,7 +25,7 @@ release-cut.
 
 ## Progress
 
-- [x] Design spec written and committed (`docs/superpowers/specs/2026-06-08-calendar-app-design.md`)
+- [x] Design spec written and committed (2026-06-08)
 - [x] V1 design decisions resolved (App name "Calendula", icon, seed color)
 - [x] Plan 01 written and executed — foundation lands (theme, icon, i18n, Hilt, DataStore, CI green)
 - [x] Plan 02 written and executed — data layer + permission flow + debug screen