Document Whitaker lint/install flow in developers guide

[coderabbitai]

Summary

`docs/developers-guide.md` still omits the Whitaker lint/install flow; the change is documented only in AGENTS.md, not the developer guide.

Required changes

Add a developers-guide section covering `make lint-clippy`, `make lint-whitaker`, `whitaker-installer` 0.2.5 installation, and the root `dylint.toml` exclusions. Sync any related design notes.

Affected areas

• `docs/developers-guide.md`
• `AGENTS.md` (for cross-reference)
• `dylint.toml`

Acceptance criteria

• Developer guide documents the full Whitaker lint/install workflow consistently with AGENTS.md.

References

• PR: $pr_url
• Requested by: @$assignee

[coderabbitai]

🔗 Related PRs

leynos/whitaker#217 - Roadmap audit [closed]
leynos/dear-diary#28 - Add Whitaker to lint checks [closed]
leynos/rstest-xfail#11 - Import df12 Rust tooling guides [merged]
leynos/limela#12 - Adopt the Whitaker Dylint suite in the lint gate and CI [merged]
leynos/catnap#22 - Adopt the Whitaker Dylint suite in the lint gate and CI [merged]

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