docs(AGENTS): add concise reproducibility recipes for build.sh

[bryan-minimal]

What

Adds a concise Reproducibility (required) subsection to the "Creating packages workflow → Step 3: Write build.sh" section of AGENTS.md (which CLAUDE.md symlinks to).

Why

Step 3's existing build.sh patterns (Autotools / CMake / Go / Rust) include no determinism flags — so a package authored by copying them defaults to non-reproducible. Reproducibility was mentioned only abstractly elsewhere in the doc (no actionable recipes). The build cache is content-addressed, so non-reproducible outputs undermine cache trust and cross-machine verification.

What's in it

A self-contained, tool-agnostic recipe per build system:

• C/C++ — -ffile-prefix-map, -gno-record-gcc-switches, -Wl,--build-id=none, ARFLAGS=Drc (+ --enable-deterministic-archives)
• Go — -trimpath -ldflags "-buildid=" (+ -buildvcs=false when a .git dir is present)
• Rust — --remap-path-prefix (build dir + cargo registry); -C codegen-units=1 + CONST_RANDOM_SEED=0 for the deeper codegen/compile-time-RNG cases
• Linux kernel — KBUILD_BUILD_TIMESTAMP/_USER/_HOST
• Embedded timestamps — SOURCE_DATE_EPOCH=0 (+ PYTHONHASHSEED=0)
• Post-install — drop libtool .la archives
• Verify — build twice, compare $OUTPUT_DIR byte-for-byte

Plus a pointer at the top of Step 3 so the patterns aren't taken as complete on their own. Links to https://reproducible-builds.org/ for background. No references to private/internal tooling.

🤖 Generated with Claude Code

Summary by CodeRabbit

• Documentation
  • Build documentation now includes explicit reproducibility requirements for all package builds
  • Provides determinism guidance across multiple build systems including C/C++, Go, Rust, Linux kernel, and embedded timestamps
  • Includes verification instructions for confirming reproducible builds through rebuild comparison

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

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📥 Commits

Reviewing files that changed from the base of the PR and between 1624946 and b72ce01.

📒 Files selected for processing (1)

• AGENTS.md

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📝 Walkthrough

Walkthrough

Adds 30 lines to AGENTS.md: a top-level requirement statement in the "Step 3: Write build.sh" section mandating reproducible builds, and a new "Reproducibility (required)" subsection enumerating determinism flags for C/C++, Go, Rust, Linux kernel, embedded timestamps, and post-install cleanup, plus a verification procedure using $OUTPUT_DIR comparison.

Changes

Reproducibility Documentation

Layer / File(s)	Summary
Reproducibility requirement and recipes AGENTS.md	Inserts a requirement statement that every build must be reproducible and a new subsection defining byte-identical output, determinism flags for multiple ecosystems (C/C++, Go, Rust, Linux kernel, embedded timestamps, post-install cleanup), and a $OUTPUT_DIR diff-based verification step.

Estimated code review effort

🎯 1 (Trivial) | ⏱️ ~3 minutes

Poem

🐇 Hop, hop, I compile again,
Same bytes each time, no random strain!
-ffile-prefix-map keeps paths tame,
SOURCE_DATE_EPOCH — timestamps the same.
Diff the trees, no surprises hide,
Deterministic builds, a bunny's pride! 🌿

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly and accurately summarizes the main change: adding reproducibility documentation/recipes to the build.sh section of AGENTS.md.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

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✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch bryan/agents-repro-section

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[msample]

LGTM. Nice!