fix(cli): fail closed on unknown --agent after remapping

[kvnloo]

Summary

Fail closed for opencode run --agent NAME instead of silently falling back to the default agent.

Exact and case-insensitive primary-agent names resolve. Unknown names and subagents exit with a useful error listing registered primary agents.

Credit / provenance

This is a current-dev refresh of upstream anomalyco#47091 by @RaviTharuma. Credit for the original diagnosis and fail-closed behavior belongs to them. This downstream refresh keeps the focused resolver/tests while adapting it to the current run-command tree.

Current refresh

• rebased cleanly onto current dev on 2026-10-03
• 0 commits behind
• only run.ts, the small agent-flag.ts helper, and its test changed
• stale unrelated lockfile / browser-opening changes were explicitly removed

Verification

The previous focused proof was 4 passing resolver tests. Current upstream CI should revalidate the refreshed head.

github_writes_origin=0 — ready to promote when upstream write access is available.

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