Restructure developer guide and synchronize agent guidance

[tkilias]

Goal

Restructure the developer documentation and keep agent-specific behavior in the repository guidance files.

Scope

• Split common, v1, and v2 developer guidance into focused documents.
• Keep human-facing workflows in the developer guide.
• Keep agent-only rules in AGENTS.md files.
• Link SLC, prerequisites and release, coverage, code-quality, and fuzzing guidance from the relevant guides.

Acceptance criteria

• The developer guide is organized into clear common, v1, and v2 sections.
• Agent-specific instructions are not duplicated in human-facing guides.
• The documentation links and repository validation checks pass.