docs: expand CONTRIBUTING.md with BST build context and AGENTS.md reference

[castrojo]

Addresses hive advisory finding from projectbluefin/common#557.

CONTRIBUTING.md was 3–5 lines with no BST context. A contributor arriving from mainline Bluefin would try to edit a Containerfile. The critical "not-bluefin" context lived only in AGENTS.md and docs/skills/not-bluefin.md, not in the human-facing CONTRIBUTING.md.

Changes

• Added prominent warning that dakota uses BuildStream 2, not Containerfiles
• Added BST prerequisites (bst, podman, just)
• Added PR target (testing branch)
• Added repo structure overview
• Added explicit pointer to AGENTS.md (where all build instructions live)

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📥 Commits

Reviewing files that changed from the base of the PR and between 2dc523b and 766e762.

📒 Files selected for processing (1)

• CONTRIBUTING.md

✨ Finishing Touches

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• Commit unit tests in branch fix/hive-contributing-update

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[github-actions]

Thanks for the PR! A maintainer will review it.

While you wait, make sure these pass locally:

just validate        # element graph check
just build default   # build the image
just boot-test       # confirm the desktop boots (exits 0 = pass)
just lint            # bootc container lint

If this PR fixes a bug, add verify steps to the linked issue so users can confirm the fix on their hardware after the next nightly ships:

```verify
ujust <something>   # what users should run to confirm the fix
```