Adds an OpenCode Free provider plugin. Free model discovery uses models.dev
(cost.input == 0 AND status != "deprecated"), matching opencode CLI's exact
filter logic.
The free tier requires a real account API key and throttles third-party
clients by User-Agent:
- With OPENCODE_FREE_API_KEY configured, the key is sent as a Bearer token
and requests identify as "opencode/latest".
- Without a key, the keyless fallback strips the SDK's always-injected empty
Authorization header and still sends the opencode User-Agent.
- The credential resolver no longer blanks OPENCODE_FREE_API_KEY
unconditionally (the stale keyless-tier assumption), and credential-pool
exhaustion no longer surfaces the misleading "Set OPENCODE_FREE_API_KEY"
message.
Co-authored-by: Jean-François <jfm@laposte.net>
Signed-off-by: Rudraksh Chahal <131520192+rudrakshchahal@users.noreply.github.com>
