feat: CLI root spans

[ajbozarth]

Background

Retired sub-issue #469 proposed CLI-level root spans. Still worth doing; low architectural complexity, mostly additive.

Scope

• Add root spans around the main CLI entry points (m serve, m alora, m decompose, m eval). Verify current CLI layout under cli/ — the original issue's file paths may be stale.
• Attributes: cli.command, cli.subcommand, mellea.cli.args (filtered to avoid leaking secrets).

Phase & dependencies

Phase 2 (coverage). Depends on Phase 1 (#1045, #1046, #1047). Lowest priority of the Phase 2 items; fully additive and architecturally simple. Can proceed in parallel with {{NEW-D}}, {{NEW-E}}, {{NEW-F}}, {{NEW-G}}.

Acceptance criteria

• CLI commands emit root spans when tracing is enabled.
• No sensitive arg values appear in attributes by default.

Parent epic: #444