Provision TOGETHER_API_KEY for optional Together provider runs

[micahstubbs]

Board issue: sia_rust-e2j.8

Finding

The local environment does not define TOGETHER_API_KEY. This is not required for the default Anthropic path or the Nebius hackathon path, but it is required if using the bundled together provider profile/config.

Code path

• docs/CREDENTIALS.md:24 documents TOGETHER_API_KEY for the bundled Together provider.
• sia/defaults/providers/together.json declares api_key_env = TOGETHER_API_KEY.
• src/llm/provider_mapping.rs:46 reads a provider's declared api_key_env for chat-completions transports.

Acceptance criteria

• Decide whether Together-backed profiles are in scope for this environment.
• If yes, add a valid key to secure local/team secret storage and the runtime environment as TOGETHER_API_KEY.
• Do not commit the secret value or paste it into GitHub.
• Verify a Together provider smoke path, attaching only non-secret output.