Fix OpenCode model discovery timeouts

[dcellison]

Summary

• replace the short-lived opencode serve model-discovery path with OpenCode's documented opencode models <provider> --refresh command
• preserve canonical effective-user authentication and configuration while removing inherited one-turn model overrides and control-plane secrets
• parse bounded metadata-only output, retain nested provider model IDs, classify failures without leaking command output, and reap cancelled commands
• replace the server-oriented tests with command-contract, isolation, safety-limit, failure-classification, and cancellation coverage

Qualification evidence

On the installed host, the command used by the new adapter completed successfully for the affected context:

$ time opencode models deepseek --refresh
deepseek/deepseek-v4-flash
deepseek/deepseek-v4-flash-vision-exp
deepseek/deepseek-v4-pro

real    0m1.239s
user    0m1.100s
sys     0m0.143s

The former temporary-server strategy timed out for the same context.

Validation

• make check
• make typecheck
• focused model-catalogue and application-host tests: 37 passed
• full suite: 6,250 passed

Closes #727