chore(ci): externalize workflow scripts and relocate main flow doc (#722)

* feat: Add GitHub Actions workflows for security audits, CodeQL analysis, contributor updates, performance benchmarks, integration tests, fuzz testing, and reusable Rust build jobs

- Implemented `sec-audit.yml` for Rust package security audits using `rustsec/audit-check` and `cargo-deny-action`.
- Created `sec-codeql.yml` for CodeQL analysis scheduled twice daily.
- Added `sync-contributors.yml` to update the NOTICE file with new contributors automatically.
- Introduced `test-benchmarks.yml` for performance benchmarks using Criterion.
- Established `test-e2e.yml` for running integration and end-to-end tests.
- Developed `test-fuzz.yml` for fuzz testing with configurable runtime.
- Created `test-rust-build.yml` as a reusable job for executing Rust commands with customizable parameters.
- Documented main branch delivery flows in `main-branch-flow.md` for clarity on CI/CD processes.

* ci(workflows): update workflow scripts and rename for clarity; remove obsolete lint feedback script

* chore(ci): externalize workflow scripts and relocate main flow doc

.github/workflows/README.md

@@ -0,0 +1,29 @@
+# Workflow Directory Layout
+
+GitHub Actions only loads workflow entry files from:
+
+- `.github/workflows/*.yml`
+- `.github/workflows/*.yaml`
+
+Subdirectories are not valid locations for workflow entry files.
+
+Repository convention:
+
+1. Keep runnable workflow entry files at `.github/workflows/` root.
+2. Keep workflow-only helper scripts under `.github/workflows/scripts/`.
+3. Keep cross-tooling/local CI scripts under `scripts/ci/` when they are used outside Actions.
+
+Workflow behavior documentation in this directory:
+
+- `.github/workflows/main-branch-flow.md`
+
+Current workflow helper scripts:
+
+- `.github/workflows/scripts/ci_workflow_owner_approval.js`
+- `.github/workflows/scripts/lint_feedback.js`
+- `.github/workflows/scripts/pr_auto_response_contributor_tier.js`
+- `.github/workflows/scripts/pr_auto_response_labeled_routes.js`
+- `.github/workflows/scripts/pr_check_status_nudge.js`
+- `.github/workflows/scripts/pr_intake_checks.js`
+- `.github/workflows/scripts/pr_labeler.js`
+- `.github/workflows/scripts/test_benchmarks_pr_comment.js`