Implement the first `ears-manager` command set and diagnostics

Plan ID

EM-04

Purpose

Deliver the first usable ears-manager CLI for local developers and CI.

Scope

Implement the command/result boundary from #30 for the minimum useful surface:

• Project/check validation.
• Requirement add, list, show, update, and retire.
• Interface add, list, and show.
• Artifact get and put.
• Minimal proposed change-set creation needed to make mutations reviewable.
• Human-readable diagnostics and deterministic JSON output.
• Stable help text, exit statuses, and failure behavior.

All writes must pass EM-03 before changing files, and failed operations must leave the working tree unchanged.

Dependencies

Depends on #30, EM-02, and EM-03.

Acceptance criteria

• Every command has documented request, success, diagnostic, and exit-status behavior.
• Golden CLI fixtures cover valid operations, malformed input, retirement, dangling references, relationship failures, and partial-operation prevention.
• JSON output is stable and contains no nondeterministic timestamps or map ordering.
• The binary works against a temporary fixture project without a service or host-language runtime.
• The first release scope and deferred commands are documented.

Out of scope

Full compare, impact analysis, and governed Git automation belong to EM-05 through EM-08.