[awf] copilot-harness: spawn /usr/local/bin/copilot ENOENT on tool-cache hit #7130

Problem
Copilot-engine workflows fail with spawn /usr/local/bin/copilot ENOENT when the runner already has copilot-cli in the tool cache, because the installer skips creating the /usr/local/bin/copilot wrapper on cache hits.

Context
github/gh-aw#51373

Root Cause
install_copilot_cli.sh's activate_cached_copilot_bin() prepends the cached dir to PATH and returns early when GITHUB_PATH is set (always true in Actions), skipping the wrapper install step (install -m 0755 wrapper_path INSTALL_DIR/copilot) that the fresh-download path performs. The compiler-emitted harness (copilot_harness.cjs) always spawns the hardcoded absolute path /usr/local/bin/copilot (constant CopilotBinaryPath), so it fails when only PATH was updated.

Proposed Solution
Either (a) have activate_cached_copilot_bin() still install/symlink the wrapper to INSTALL_DIR/copilot even when GITHUB_PATH is set, or (b) make the harness resolve copilot via PATH lookup instead of a hardcoded absolute path, falling back to the constant only if PATH resolution fails. Note any AWF sandbox interaction with /usr/local/bin mount visibility for cached binaries.

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