docs: update AGENTS.md with comprehensive agent instructions

Update AGENTS.md to give future agent sessions a compact, verified set of high-signal instructions for working in this repo.

What this MR does

Replaces the previous AGENTS.md with content focused on details that an agent could not infer from filenames or generic Go conventions.

High-signal content kept or added

• Project layout: command grammar, real entrypoints, and the internal/cmdutils import boundary enforced by depguard.
• Lefthook flow: make bootstrap + mise + .tool-versions, skip env vars, and the exact lefthook run pre-push checks.
• Test gotchas: make test clears VISUAL/EDITOR/PAGER/GITLAB_TOKEN and sets CI_PROJECT_PATH; integration tests use //go:build integration, the _Integration suffix, make integration-test-race, and require GITLAB_TEST_HOST + GITLAB_TOKEN_TEST.
• Generated artifacts: docs/source/ is generated; pre-commit and pre-push hooks fail on drift for both make gen-docs and make generate.
• Lint rules that actually bite: PrintJSON over json.Marshal, the cmdutils import boundary, and golangci-lint run --new-from-rev=origin/main on pre-push.
• Command conventions: unexported options struct, optional newOptions, partial complete/validate/run implementation, Conventional Commits.
• Skills validator command and the failure modes it catches.
• Environment variables that change behavior, with the GITLAB_HOST / GITLAB_URI / GL_HOST aliases and GLAB_ENABLE_CI_AUTOLOGIN + GITLAB_CI pairing.