fix(cli): keep legacy hidden selectors valid for advertise-required inputs

[nolanchic]

Summary

Since #2004, Registry.Invoke validates builtin commands against their advertised schema before the handler runs. Three agentcontext commands mark agent-id as advertise-required, so the schema lists it in required — but the daemon binder deliberately treats it as optional: resolveAgentSelector accepts exactly one of --agent-id or the hidden deprecated --provider (#2193). The pre-binding validator therefore rejected the supported legacy spelling outright, and passing both selectors tripped the binder's exactly-one guard — no valid spelling was left for that path.

The AdvertisedRequired doc comment already states the intent — keep the advertised contract strict while allowing a hidden compatibility selector to satisfy runtime validation — so the fix says exactly that in the schema itself:

• a new advertise-alt struct tag names the hidden fields that can stand in for an advertise-required field (the three commands declare advertise-alt:"provider" on agent-id)
• Schema() emits one anyOf branch per accepted spelling ({type:"object", required:["agent-id"]}, {type:"object", required:["provider"]}) instead of a hard required entry. One branch per name matters — a single branch listing both names would require both at once. Typed object branches are required because the invocation validator only applies required to branches declaring an object type.
• fields without an alternate render exactly as before, so no other advertised contract changes

Notes

• Passing both selectors still reaches the handler, which rejects it with the exactly-one guard and its hint message — that error path is unchanged.
• Cosmetic: help-text generation (command_catalog.go) reads the top-level required list, so agent-id now shows as optional in generated help. The anyOf constraint is not reflected there; happy to follow up if the help generator should learn about alternates.

Test plan

• TestStartCommandSchemaPassesInvocationValidationForLegacySelector e2e — the real start command through NewRegistryFromProviders → Invoke: canonical and legacy spellings pass the pre-binding validator, neither present is rejected
• TestSchemaRendersAnyOfForAdvertisedRequiredWithAlternate — one branch per spelling, hidden field stays out of properties
• TestSchemaKeepsHardRequiredForAdvertisedRequiredWithoutAlternate — no-alternate fields unchanged
• TestRegistryInvokeAcceptsLegacyAlternateForAdvertisedRequired — validator-level coverage
• Updated TestStartCommandRequiresOneSelectorAndPrompt / TestAgentStartCommandAllowsOmittedModel to the new advertised shape
• go test ./service/cli/... — 11 packages ok; full ./service/... — 42 packages ok; gofmt clean; ValidateCapabilityInputSchema accepts the new shape

Fixes #2193