fix(server): align logging, introspection, completion, and preset flags #1448

Parent

• epic(server): close llama-server b10621 compatibility gaps #1431

Problem

b10621 provides cache listing, Bash completion generation, log file/color/verbosity/prefix/timestamp controls, prompt-directory logging, and several built-in model presets. mlxcel overlaps only part of this surface, uses a noncanonical log-file environment variable, and must avoid accepting observability options whose output format or security behavior differs.

Scope

• Align --log-disable, --log-file, colors, verbose/verbosity, prefix, timestamp, and canonical environment bindings.
• Define log destination precedence and rotation/error behavior without silently losing logs.
• Implement secure --log-prompts-dir handling or reject it; include permissions, redaction policy, atomic files, and retention limits.
• Implement or explicitly reject --cache-list and --completion-bash with stable, testable output.
• Classify built-in EmbeddingGemma, FIM Qwen, GPT-OSS, vision Gemma, and speculative preset flags against mlxcel's direct checkpoint support.
• Keep human logs separate from machine-readable HTTP/SSE responses.

Acceptance criteria

• Each logging and introspection option has exact CLI/environment/default tests.
• Log files and prompt logs use restrictive permissions and never contain API keys or repository tokens.
• Invalid/unwritable destinations fail at startup instead of falling back silently.
• Shell completion output is syntax-checked and includes visible compatibility options without exposing hidden unsafe aliases.
• Preset flags either resolve to documented immutable checkpoint identifiers or fail with an explicit download example.