refactor(yamllint): rename config to .yamllint.yml and match published standard

[sbaerlocher]

Summary

• Rename .yamllint to .yamllint.yml via git mv, matching the name this repository publishes in rst/guide/best-practices/standards.rst, rst/guide/development/cicd.rst and rst/guide/getting-started/environment.rst
• Align the two diverging values with the published standard: line-length.max 160 to 500, and "on" added to truthy.allowed-values. The published text is unchanged; the config follows it
• Pull the two references along in the same commit (lefthook.yml, AGENTS.md) so the pre-commit hook stays resolvable at every point in history

All 16 rules, all 10 ignores (including the repo-specific build/, temp-rst/, rst/collections/) and line-length.level: warning are kept.

Test plan

• yamllint -c .yamllint . before the change: exit 0, two document-start warnings (docker-compose.yml, .dde/config.yml)
• yamllint -c .yamllint.yml . after the change: exit 0, the same two warnings — behaviour-neutral. Both warnings pre-date this change
• No remaining reference to the old filename: rg '\.yamllint(?!\.yml)' --pcre2 returns nothing
• git diff --cached --stat -M shows .yamllint => .yamllint.yml as a rename, not delete plus add
• lefthook pre-commit ran on commit (gitleaks, yamllint, prettier, markdownlint all green), proving lefthook.yml resolves the new path

[claude]

No issues found.

Verified this is a clean, well-scoped refactor:

• Rename .yamllint → .yamllint.yml (an auto-detected yamllint config name, so discovery keeps working).
• line-length.max 160→500 and truthy.allowed-values +on both match the published standard in rst/guide/development/cicd.rst and rst/guide/getting-started/environment.rst. Both are relaxations, so no previously-passing file can newly fail.
• References in lefthook.yml and AGENTS.md updated; no stale .yamllint filename references remain anywhere in the repo.