ci: remove old .gitea workflows - use .forgejo only

The .gitea/ workflows were outdated and caused conflicts:
- Used runs-on: rust, golang (non-existent runners)
- Complex docker:27-cli setup with TLS (fragile)
- Different secret variable names (FORGEJO_REGISTRY_TOKEN vs REGISTRY_PAT)
- No tests before build

.forgejo/workflows/build.yaml is the clean, working version:
- Simplified docker commands
- Proper runner: docker
- Tests run first
- Cleanup on failure
- No hanging processes

fixtures/query-bad-charset.yaml

@@ -1,10 +0,0 @@
-project: poimen
-roots:
-  - /Users/rockliang/workplace/Poimen/agent-rust
-sources: [pi, claude]
-queries:
-  - id: infra/root-causes
-    question: What infrastructure bugs were found?
-defaults:
-  memory_budget: 1024
-  chunk_tokens: 5000