Add historical benchmark storage with orphan branch and relative regression detection

[Mossaka]

Problem

Each benchmark run is isolated — no trending, no relative regression detection. A 50% slowdown that stays under the critical threshold goes unnoticed. Two prior PRs (#1720, #1767) attempted this but were closed without merging. The branch feat/240-benchmark-history-tracking has solid prior art to build on.

Approach

1. Orphan benchmark-data branch stores benchmarks/history.json (last 50 entries)
2. Workflow steps: fetch history → run benchmarks → append + trim → commit to orphan branch
3. Relative regression detection: flag when current p95 exceeds rolling mean of last 10 runs by >25%
4. Permissions: contents: read → contents: write (push to benchmark-data branch only)

New Files

• scripts/ci/benchmark-trend.ts — trend report generator (adapt from feature branch)
• scripts/ci/benchmark-utils.ts — shared stats/parsing utilities
• scripts/ci/benchmark-utils.test.ts — unit tests

Modified Files

• scripts/ci/benchmark-performance.ts — load previous run, add delta regression check
• .github/workflows/performance-monitor.yml — add history fetch/commit/trend steps
• package.json — add benchmark:trend script

[Mossaka]

Part of benchmark epic #240. Supersedes #1760. Builds on prior art from closed PRs #1720 and #1767 (branch feat/240-benchmark-history-tracking).

[github-actions]

👋 Hello! I noticed this issue might be related to an existing open issue:

• [plan] Add historical benchmark storage and relative regression detection #1760 — [plan] Add historical benchmark storage and relative regression detection — Describes the same core goal: storing benchmark history and detecting relative regressions (>25% slowdown vs. rolling mean). The files to modify are identical (benchmark-performance.ts, performance-monitor.yml), and the acceptance criteria overlap significantly. The main difference is the storage mechanism (Actions Cache in [plan] Add historical benchmark storage and relative regression detection #1760 vs. orphan branch here).

Since #1760 is still open, you may want to close it as superseded by this issue (or vice versa) to avoid parallel tracking.

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