docs: add developer call-to-action and wire it into contribution docs

[welshDog]

Motivation

• Make contributor onboarding more actionable by providing a focused, high-impact call-to-action that maps expertise areas to concrete ways to help.
• Surface prioritized contribution areas that align with the project's neurodivergent-first design and AI-native goals to reduce ambiguity for new collaborators.

Description

• Added a new guide at docs/DEVELOPER_CALL_TO_ACTION.md that outlines target contribution areas (architecture, DevOps, AI integration, accessibility, language features, and future-hardware research) and contribution paths.
• Updated README.md to link to the core contributing guide and the new developer call-to-action for easier discovery.
• Updated .github/CONTRIBUTING.md to include a High-Impact Contribution Areas section pointing contributors to the new call-to-action document.

Testing

• Ran a small python verification script that asserted the presence of the new file and that README.md/.github/CONTRIBUTING.md reference it, and the check succeeded.
• Validated the repository change summary and file list to confirm the new file and markdown updates were included, and the validation succeeded.

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Codex Task