Update spec links to docs.megaeth.com/spec and add mega-evme doc links

[Troublor]

Summary

• Migrate all spec cross-links from the old app.gitbook.com/o/.../s/.../ URLs to the new https://docs.megaeth.com/spec/ base URL across 13 files.
• Fix subpage prefix: evm/ → megaevm/ to match the live spec site structure (verified by fetching every URL).
• Also replace the intermediate docs.megaeth.com/evm-spec/ pattern used in docs/dev/AGENTS.md.
• Add mega-evme doc links (https://docs.megaeth.com/mega-evme) to the debugging page and dev overview where the tool is already referenced.
• Update AGENTS.md (repo-root and docs/) to document the new spec URL convention and mega-evme linking pattern.

[claude]

LGTM. Clean, complete URL migration across all 13 files — no old app.gitbook.com or evm-spec/ links remain. The megaevm/ vs system-contracts/ prefix split is applied correctly and is now documented in both AGENTS.md files (note: CLAUDE.md is a symlink to AGENTS.md, so the convention update covers both automatically). The scoped mega-evme link additions in debugging.md and dev/overview.md are sensible.