feat: replace RUNNER_TEMP with /tmp/gh-aw throughout compiler and runtime

[Copilot]

Summary

Updates the compiler and all built-in workflow templates to never use ${RUNNER_TEMP}/gh-aw/ as the runtime root directory. All paths now use /tmp/gh-aw/ directly.

Why

• /tmp/gh-aw/ is always available on Linux runners (GitHub-hosted and self-hosted)
• Eliminates dependency on RUNNER_TEMP environment variable which can vary or be unexpectedly set to /tmp on some self-hosted runners
• Simplifies the security model by removing the old two-directory split between ${RUNNER_TEMP}/gh-aw/ (setup files) and /tmp/gh-aw/ (runtime files)

Changes

Core constants

• pkg/constants/constants.go: Updated GhAwRootDir and GhAwRootDirShell to /tmp/gh-aw

Compiler (Go files)

All compiler files updated to emit /tmp/gh-aw/ paths instead of ${RUNNER_TEMP}/gh-aw/ or ${{ runner.temp }}/gh-aw/:

• pkg/workflow/compiler_yaml.go, compiler_yaml_ai_execution.go, compiler_yaml_main_job.go, compiler_yaml_step_generation.go
• pkg/workflow/mcp_config_builtin.go, mcp_renderer_builtin.go, mcp_renderer.go, mcp_setup_generator.go, mcp_scripts_generator.go
• pkg/workflow/cache.go, docker.go, runtime_step_generator.go, repo_memory.go
• pkg/workflow/awf_helpers.go, copilot_engine_execution.go, notify_comment.go, pr.go
• pkg/workflow/git_configuration_steps.go, safe_jobs.go, redact_secrets.go, publish_artifacts.go
• pkg/workflow/unified_prompt_step.go, step_order_validation.go

Setup action

• actions/setup/setup.sh: Hardcoded GH_AW_ROOT="/tmp/gh-aw", removed all RUNNER_TEMP validation (collision check, writability check, etc.)
• actions/setup/clean.sh: Updated default destination
• actions/setup/action.yml: Updated input description
• actions/setup/sh/*.sh: Updated all shell scripts
• actions/setup/js/*.cjs: Updated all JavaScript runtime files and test files

Generated files

• All 182 .github/workflows/*.lock.yml files recompiled with new paths
• All pkg/workflow/testdata/**/*.golden wasm golden files updated

Testing

• All unit tests pass (go test -tags '!integration' ./pkg/workflow/ ✓)
• Build succeeds (make build ✓)
• Lint passes (make lint ✓)
• Format passes (make fmt ✓)
• All 182 workflows successfully recompiled (make recompile ✓)