feat: integrate tea CLI for forge issue/label/comment operations (#666)

- Add lib/tea-helpers.sh with tea_file_issue, tea_relabel, tea_comment,
  tea_close — thin wrappers preserving secret scanning on write ops
- Add tea 0.9.2 binary to docker/agents/Dockerfile
- Configure tea login in docker/agents/entrypoint.sh from FORGE_TOKEN/FORGE_URL
- Derive TEA_LOGIN in lib/env.sh (codeberg vs local forgejo)
- Source tea-helpers.sh conditionally when tea binary is available
- Migrate predictor formula from inline curl to tea CLI commands
- Register tea-helpers.sh in smoke test function resolution

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

docker/agents/Dockerfile

@@ -4,6 +4,10 @@ RUN apt-get update && apt-get install -y --no-install-recommends \
     bash curl git jq tmux cron python3 openssh-client ca-certificates \
     && rm -rf /var/lib/apt/lists/*
 
+# tea CLI — official Gitea/Forgejo CLI for issue/label/comment operations
+RUN curl -sL https://dl.gitea.com/tea/0.9.2/tea-0.9.2-linux-amd64 -o /usr/local/bin/tea \
+    && chmod +x /usr/local/bin/tea
+
 # Claude CLI is mounted from the host via docker-compose volume.
 # No internet access to cli.anthropic.com required at build time.