feat(online): report terminal agent readiness by account

[AetherAI3]

Summary

• Probe the authenticated, owner-scoped Cloud terminal-readiness contract on every agent list, show, and chat call. Report registry, DM, and model/UVT admission separately in human and JSON output, with safe stable codes for account disablement, wrong credential class, incompatible contracts, and outages. No readiness is cached across token rotation or account switch.
• Add a read-only terminal readiness check to aether doctor --live; fast doctor explicitly leaves remote account readiness unverified. Local coding paths remain independent of this probe.
• Treat a saved DM as admitted only when Cloud returns admitted or replied; refused or unavailable admissions are labelled as saved but not accepted, and a one-shot send exits nonzero. Conversation sync still follows a successful read.
• Validate readiness and admission vocabulary before printing it, so server text cannot leak credential or conversation detail. An unspecified HTTP 403 is reported as refused rather than assuming account disablement.

Verification

• Exact Agent head e2c89e3a1435ddc4f2bc84fc33bec8d5998ba0e3 (base f9a2bd07a6ee61d33e46df12f343b2170c484cca): AETHER_ATS_PYTHON set to installed Python 3.11; npm test passed 2,875 tests, 17 skips, 0 failures. The skips are environment-dependent. Focused managed-agent, ATS, and doctor tests passed 105 tests, 3 Windows symlink privilege skips, 0 failures. git diff --check passed.
• Companion Cloud head 08d53d5a06c4c160a352eb159eb4a6ace02828d8 merged as AetherAI3/AETHER-CLOUD#1879 (dd5a9bfa2221b5839b4a669dae57649d85f9a2b1). Its exact-head managed-agent, collaboration, and OpenAPI suite passed 1,252 tests, with 2 pre-existing warnings.
• GitHub Actions CI and CodeQL checks are marked failed because jobs were not started: the check annotation says, "The job was not started because your account is locked due to a billing issue." Aether Hosted Build & Test is neutral. These are not hosted test passes; local results above are the verification evidence.

Scope

This is terminal Online readiness and diagnostics. The server remains authoritative for each operation and message admission. ATS execution, RC browser control, and deployed account canaries remain separate gates.

Addresses #234.