fix: reduce Smoke Copilot context size to fix safe_outputs invocation

Full repo checkout causes Copilot CLI to ingest 1M tokens of context
(via --add-dir "${GITHUB_WORKSPACE}"), making it use bash commands
instead of MCP tools. The safeoutputs-add_comment tool is never called,
outputs.jsonl is never created, and the safe_outputs job always skips.

Fix: Remove explicit actions/checkout from smoke-copilot.md (compiler
generates sparse checkout by default). Update the postprocess script to:
1. Convert sparse checkout → full checkout (needed for npm ci/build)
2. Add a "Clean workspace for agent" step that removes src/, tests/,
   etc. after building, keeping only .github, .agents, dist, node_modules

This reduces Copilot's context from ~1M to ~200k tokens, allowing it
to properly use MCP safe-output tools.

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

Author: Mossaka