meta: contract freeze policy and 1.0 gate checklist

[jamiesun]

Background

Agent ecosystems punish interface drift. The versioned schemas (sshx.request.v1 / sshx.result.v1 / sshx.event.v1) already exist; what's missing is a public commitment about what is frozen and when.

Plan

• Author a contract-stability document (docs/contract.md): enumerate the frozen surface — CLI flags per subcommand, JSON result fields, error_kind values, exit codes, JSONL event types, audit event fields.
• Define the compatibility policy: additive-only within v1 schemas; breaking change ⇒ v2 schema + N-1 support window.
• Define the 1.0 gate checklist (e.g. MCP stdio shipped, Windows E2E green, keyringstore covered, no known contract-breaking TODOs) and burn it down.

Acceptance

• docs/contract.md merged and linked from README + AGENT.md
• CHANGELOG "Unreleased" section notes the freeze policy
• 1.0 milestone created with gate issues attached