docs: streamline guides and clarify agent safeguards

[jiminu]

The environment guide mixed everyday usage with a long debugging reference, while terminal setup and contributor instructions were spread across multiple documents. Move debugging into its own linked guide, consolidate terminal setup in Installation, and separate contributor procedures from architecture details while preserving existing heading anchors and instructions.

Clarify reinstall guidance and agent rules for prerequisite reading, mise cleanup exclusions, and preservation of unused Neovim checkouts. Consolidate repeated agent guidance and document the existing generated doc/tags exception. Runtime behavior is unchanged.

Validation:

• git diff --check origin/main...HEAD
• Checked 15 Markdown files: 79 local link targets, 49 heading anchors, balanced code fences, and retained existing headings.
• Compared relocated debugging text and code examples, managed-state documentation, and the latest upstream Go policy/Git UI documentation for preservation.
• Full repository gate not run locally: documentation-only changes use focused validation under AGENTS.md. GitHub CI results are reported separately by the PR checks.