extract reusable command contract helpers for sibling products

[autholykos]

Context

rlm-tender should follow the same automation posture as Pituitary:

• machine-readable JSON;
• --request-file PATH|-;
• command schemas;
• normalized request/result/issue envelopes;
• stale-state preflight;
• content-trust metadata.

Pituitary already has these patterns, but much of the implementation lives under internal/.

Proposed Capability

Extract a small public package for command/operation contract helpers that sibling products can reuse.

Candidate contents:

• Response[Req, Res];
• Issue;
• request-file parsing helpers;
• JSON Schema registry helpers;
• format negotiation helpers;
• timing/content-trust metadata helpers, if appropriately generic.

Non-Goals

• No spec/doc semantics.
• No Pituitary-specific commands.
• No dependency on Pituitary config.

Acceptance Criteria

• A sibling product can import the package without importing Pituitary internals.
• Existing Pituitary commands continue to use the extracted helpers.
• The package has tests independent of spec/doc analysis.