Add ROADMAP_TASKS.md: Codex-ready roadmap task breakdown

[Intina47]

Motivation

• Turn the high-level roadmap into actionable, parallelizable tasks so Codex can start implementing workstreams directly.
• Make goals, scope, deliverables, and acceptance criteria explicit to reduce ambiguity and speed implementation across Streams A–G.

Description

• Add ROADMAP_TASKS.md which breaks the roadmap into Streams A–G and defines tasks A1–G1 with clear goals, scope, deliverables, and acceptance criteria.
• Each task is written as a Codex-friendly prompt and includes CLI examples and expected behaviors for implementation and verification.
• This is a documentation-only change and does not modify runtime code or build artifacts.

Testing

• No automated tests were added or executed because this change only adds documentation.
• The change is expected to be a no-op for existing CI and unit test suites.

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Codex Task