Unrelated pending edits: CI config, AGENTS.md, NativeBuildPlugin doc

[jbachorik]

What does this PR do?:
Splits out three pre-existing local modifications that were unrelated to the
reference-chains work in progress on another branch, so they can be reviewed
independently:

• .gitlab/reliability/.gitlab-ci.yml
• AGENTS.md
• doc/build/NativeBuildPlugin.md

Motivation:
These edits were sitting in the working tree from prior, separate work and
shouldn't be bundled into the reference-chains PR.

Additional Notes:
No functional relationship between the three files beyond "all pending at
the same time" - please review each independently.

How to test the change?:
Docs/CI-config only; no code changes.

For Datadog employees:

• If this PR touches code that signs or publishes builds or packages, or handles
  credentials of any kind, I've requested a security review (run the dd:platform-security-review
  skill, or file a request via the PSEC review form).
  bewaire also runs automatically on every PR.
• This PR doesn't touch any of that.
• JIRA: [N/A]

Unsure? Have a question? Request a review!

🤖 Generated with Claude Code

[dd-octo-sts]

CI Test Results

Run: #29035344857 | Commit: 56e5adf | Duration: 17m 0s (longest job)

✅ All 32 test jobs passed

Status Overview

JDK	glibc-aarch64/debug	glibc-amd64/debug	musl-aarch64/debug	musl-amd64/debug
8	-	✅	-	-
8-ibm	-	✅	-	-
8-j9	✅	✅	-	-
8-librca	-	-	✅	✅
8-orcl	-	✅	-	-
11	-	✅	-	-
11-j9	✅	✅	-	-
11-librca	-	-	✅	✅
17	✅	✅	-	-
17-graal	✅	✅	-	-
17-j9	✅	✅	-	-
17-librca	-	-	✅	✅
21	✅	✅	-	-
21-graal	✅	✅	-	-
21-librca	-	-	✅	✅
25	✅	✅	-	-
25-graal	✅	✅	-	-
25-librca	-	-	✅	✅

Legend: ✅ passed | ❌ failed | ⚪ skipped | 🚫 cancelled

Summary: Total: 32 | Passed: 32 | Failed: 0

---

Updated: 2026-07-09 17:19:03 UTC

[chatgpt-codex-connector]

💡 Codex Review

Here are some automated review suggestions for this pull request.

Reviewed commit: e7ae433755

ℹ️ About Codex in GitHub

Your team has set up Codex to review pull requests in this repo. Reviews are triggered when you

• Open a pull request for review
• Mark a draft as ready
• Comment "@codex review".

If Codex has suggestions, it will comment; otherwise it will react with 👍.

Codex can also answer questions or update the PR. Try commenting "@codex address that feedback".

[chatgpt-codex-connector]

Restore the missing JDK upgrade guide

This replacement points the removed JDK upgrade checklist to doc/build/JdkUpgrades.md, but that file is not present in doc/build and rg JdkUpgrades only finds this new link. As a result, the detailed build/target JDK upgrade instructions deleted from AGENTS.md are no longer reachable for anyone following the project guidance.

Useful? React with 👍 / 👎.