perf(ci): reduce GitHub Actions costs ~60-65% across all workflows

Analysis of Feb 17 data showed 400+ workflow runs/day consuming ~398 billable minutes (~200 hours/month projected). Implemented targeted optimizations:

High-impact changes:

- sec-audit.yml: add path filters (Cargo.toml, src/**, crates/**, deny.toml); skip docs-only PRs

- test-benchmarks.yml: move from every-push-to-main to weekly schedule; retention 30d -> 7d

- pub-docker-img.yml: tighten PR smoke build path filters to Docker-specific files only

- sec-codeql.yml: reduce from twice-daily (14 runs/week) to weekly

Medium-impact changes:

- ci-run.yml: merge lint + lint-strict-delta into single job; drop --release from smoke build

- feature-matrix.yml: remove push trigger (weekly-only); remove redundant cargo test step

- dependabot.yml: monthly instead of weekly; reduce PR limits from 11 to 5/month; group all deps

Runner cost savings:

- Switch 6 lightweight API-only workflows to ubuntu-latest (PR Labeler, Intake, Auto Responder, Check Stale, Check Status, Sync Contributors)

- pr-check-status.yml: reduce from every 12h to daily

New files:

- docs/ci-cost-optimization.md: comprehensive analysis and revised architecture documentation

- scripts/ci/fetch_actions_data.py: reusable GitHub Actions cost analysis script

Estimated impact: daily billable minutes ~400 -> ~120-150 (60-65%% reduction), monthly hours ~200 -> ~60-75, Dependabot PRs ~44/month -> ~5 (89%% reduction)

.github/workflows/pr-check-stale.yml

@@ -12,9 +12,7 @@ jobs:
         permissions:
             issues: write
             pull-requests: write
-        runs-on: blacksmith-2vcpu-ubuntu-2404
-        steps:
-            - name: Mark stale issues and pull requests
+        runs-on: ubuntu-latest
               uses: actions/stale@b5d41d4e1d5dceea10e7104786b73624c18a190f # v10.2.0
               with:
                   repo-token: ${{ secrets.GITHUB_TOKEN }}