STAT-115-remove-check-shared-block: Remove claude/check-shared-block composite action

Superseded by a live @../stat-tackler-claude/CLAUDE.md import, which
means there is no per-repo copy left to drift and nothing left to check
(part of STAT-115).

Claude-Session: https://claude.ai/code/session_01FS9R1ZEc8Vu7hg2PmzqbpP

README.md

@@ -4,12 +4,6 @@ Reusable composite actions for Gitea CI/CD pipelines.
 
 ## Actions
 
-### Claude
-
-| Action | Description |
-|---|---|
-| [claude/check-shared-block](claude/check-shared-block/README.md) | Fails when a repo's shared CLAUDE.md block has drifted from the canonical copy in stat-tackler-infra |
-
 ### Docker
 
 | Action | Description |