ci: add agent guide and benchmark comparison

[yangzhg]

Summary

• Add a repository-level AGENTS.md guide for coding agents and contributors.
• Add benchmark comparison CI that runs base/head Google Benchmark jobs, compares JSON outputs, and posts a sticky PR comment.
• Add scripts/tools/compare-benchmark.py to generate Markdown benchmark summaries and fail on regressions above the configured threshold.

Notes

• This PR changes no library runtime code.
• The benchmark workflow is intended for follow-up OOM split PRs; once this lands on master, later PRs can consume it directly.

Validation

• python3 -m py_compile scripts/tools/compare-benchmark.py
• python3 scripts/tools/compare-benchmark.py /tmp/sonic_master_bench.json /tmp/sonic_master_bench.json --warn-threshold=3 --fail-threshold=5 --output=/tmp/sonic_benchmark_comment_test.md
• git diff --cached --check before commit

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 69.67%. Comparing base (4250a05) to head (591afe4).
⚠️ Report is 25 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #137      +/-   ##
==========================================
- Coverage   74.79%   69.67%   -5.13%     
==========================================
  Files          21       27       +6     
  Lines        2436     3706    +1270     
  Branches      667     1189     +522     
==========================================
+ Hits         1822     2582     +760     
+ Misses        297      211      -86     
- Partials      317      913     +596     

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[Copilot]

Pull request overview

This PR adds contributor/agent guidance and introduces CI automation to run and compare Google Benchmark results between a PR’s base and head commits, then publish the comparison as a sticky PR comment.

Changes:

• Added a root-level AGENTS.md describing project structure, workflows, and contributor/agent guidance.
• Added a benchmark comparison workflow that runs Bazel benchmarks on base/head, generates a Markdown summary, and uploads it as an artifact.
• Added a follow-up workflow that downloads the benchmark artifact and posts/updates a sticky PR comment with the results.

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 4 comments.

Show a summary per file

File	Description
scripts/tools/compare-benchmark.py	New Python tool to diff two Google Benchmark JSON outputs and generate a Markdown summary.
AGENTS.md	New repository-level guide for agents and contributors (project shape, commands, conventions).
.github/workflows/test_coverage.yml	Splits Codecov upload behavior between PR vs push events and enables OIDC for push uploads.
.github/workflows/benchmark_compare.yml	New workflow to run base/head benchmarks, compare outputs, and upload artifacts.
.github/workflows/benchmark_comment.yml	New workflow to post/update a sticky PR comment based on the benchmark artifact.

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