docs(agents): adopt kernel's AI coding assistants policy

Align LocalAI with the Linux kernel project's policy for AI-assisted
contributions (https://docs.kernel.org/process/coding-assistants.html).

- Add .agents/ai-coding-assistants.md with the full policy adapted to
  LocalAI's MIT license: no Signed-off-by or Co-Authored-By from AI,
  attribute AI involvement via an Assisted-by: trailer, human submitter
  owns the contribution.
- Surface the rules at the entry points: AGENTS.md (and its CLAUDE.md
  symlink) and CONTRIBUTING.md.
- Publish a user-facing reference page at
  docs/content/reference/ai-coding-assistants.md and link it from the
  references index.

Assisted-by: Claude:claude-opus-4-7