fix(cli): validate boolean cache flags, fix CLI-over-env precedence

[inureyes]

Summary

Tightens the boolean cache-flag CLI contract, fixes a CLI-over-env precedence bug for those flags, and cleans up the affected --help text.

Changes

Flag validation

--prompt-cache-enabled and --apc-enabled (on both the mlxcel-server binary and the mlxcel serve subcommand) now use require_equals + num_args = 0..=1 + default_missing_value = "true" + ArgAction::Set. This gives one well-defined contract: bare --prompt-cache-enabled means true, --prompt-cache-enabled=false disables it, and a stray space-separated --prompt-cache-enabled false is rejected with a clear parser error instead of silently keeping the default and treating false as an unexpected argument.

CLI-over-env precedence (bug fix)

A new long_cli_flag_was_set helper (defined in server::cli_input, re-exported from server) scans the process argv for an explicit long flag. Startup previously passed a hardcoded false for "was this set on the CLI", so the env-var fallback was always consulted and an explicit --prompt-cache-enabled=false / --apc-enabled=false could be silently overridden by MLXCEL_PROMPT_CACHE_ENABLED / APC_ENABLED. Both binaries now pass long_cli_flag_was_set(...), so an explicit CLI value wins while the compiled-in default stays overridable by env when the flag is absent.

Help-text cleanup

• Removed stale internal tracker identifiers from the --help surface and option doc comments on the touched flags.
• Repointed two stale doc links to this repository's layout: docs/distributed.md for the pipeline-parallelism workflow and the project README for the model-surgery operations. (One identical pre-existing broken link at the --pp-auto option was corrected in the same file for consistency.)
• Added the pipeline_tensor_parallel value to the --node-role help list so it matches the roles the server accepts.

Tests

Updated the #[ignore]d prompt-cache integration tests (prompt_cache_e2e, prompt_cache_prefill_bench) to invoke --prompt-cache-enabled=true|false with the now-required = syntax; their previous space-separated form would fail to parse under the tightened contract.

Out of scope

A version bump and the en/ko docs + manpages from the originating change are intentionally excluded: this repository uses a flat docs/ tree with no corresponding files, and the version is left at 0.0.29.

Verification

• cargo fmt --all -- --check — clean
• cargo clippy --features metal,accelerate --lib --bin mlxcel --bin mlxcel-server --tests -- -D warnings — clean
• Built mlxcel-server and confirmed parsing:
  • --prompt-cache-enabled=false → parses (then fails at model load, i.e. past arg parsing)
  • --prompt-cache-enabled false → rejected: 'false' cannot be used with '--prompt-cache-enabled[=<BOOL>]'
  • --prompt-cache-enabled (bare) → parses via default_missing_value