UPSTREAM PR #17993: CLI: fixed dead links to tools/main for cli and completion, fixed code owners

[loci-dev]

Mirrored from ggml-org/llama.cpp#17993

Let's start the separation of cli and completion tools.

1. Fixed dead links for tools/main.
2. Mentioned cli and completion together where the user can use both tools.
3. Fixed code owners for cli and completion tools.
4. Added README.md to tools/cli with TODO.
5. Used llama-completion instead of llama-cli in tools/completion README.

PR is related to #17824.

Thank you.

[loci-review]

Explore the complete analysis inside the Version Insights

Performance Analysis Summary - PR #550

Analysis Type: Documentation-only changes
Scope: 7 markdown files modified (+28/-24 lines)
Performance Impact: None

This PR updates documentation references from the deprecated tools/main directory to the separated tools/cli and tools/completion directories. No source code, binaries, or performance-critical functions were modified. All changes are limited to markdown documentation files (CODEOWNERS, README.md, grammars/README.md, and tool-specific documentation).

Power consumption analysis confirms zero change across all 16 binaries. No functions within Performance-Critical Areas (matrix operations, attention mechanisms, memory management, quantization kernels, or backend dispatch) were affected. The inference pipeline functions (llama_decode, llama_encode, llama_tokenize) remain unchanged, resulting in no impact on tokens per second throughput.