feat: add direct Anthropic API authentication and completions

[danielkov]

Summary

Add direct Anthropic API completions using AgentKit’s existing Anthropic adapter, with kit auth login|status|logout anthropic, ANTHROPIC_API_KEY, and provider/model selection across CLI, TUI, and ACP.

Motivation

Claude Max subscribers can claim API credits into a Claude Console organization and use that organization’s API key with Kit. Direct API requests draw from Anthropic billing without using Claude subscription OAuth credentials.

Impact

Existing provider defaults remain unchanged. Anthropic login accepts a single-workspace API key at a hidden prompt, validates it through the non-inference Models API, and stores it in the selected persistent credential backend. A nonempty environment key overrides the saved key; logout removes only local credentials.

Document direct API setup and credential behavior. Credit balances remain in Claude Console rather than kit usage.

Technical details

Native completion adapter

Pin agentkit-provider-anthropic to 0.10.11; no transitive package versions change. The published archive checksum and packaged source match registry metadata and the upstream commit; provenance remains sole-owner and commit-based without a matching release tag.

Discover models through the authenticated, paginated Anthropic Models API. Resolve selected IDs and aliases through the model-detail endpoint; use returned context limits, output ceilings, and thinking capabilities instead of hardcoded model-name rules. Requests are bounded by time and response size with redirects disabled. Preserve explicitly selected models with default reasoning when discovery is unavailable.

Cross-provider sessions

Remove unsigned foreign reasoning only from the Anthropic-bound transcript projection while preserving Anthropic signed and redacted thinking blocks. This permits switching existing sessions to Anthropic without sending invalid thinking blocks.

[kit-code-agent]

No issues found. The changes look good to merge.

[kit-code-agent]

No issues found. The changes look good to merge.

_{Took 1 min 43 sec}

[kit-code-agent]

No issues found. The changes look good to merge.

_{Took 2 min 37 sec}

[kit-code-agent]

No issues found. The changes look good to merge.

_{Took 3 min 51 sec}

[kit-code-agent]

No issues found. The changes look good to merge.

_{Took 3 min 6 sec}