Add DeepSeek and MiniMax gateway lanes

[thisguymartin]

Goal

Add DeepSeek and MiniMax gateway lanes to pstack-flex while keeping the stock model routes and upstream content intact.

Scope

• Run each gateway lane through the external runner with its provider key, endpoint, and isolated Claude configuration.
• Remove inherited Anthropic headers and cloud-provider routing before starting a gateway child.
• Reject a reported model mismatch. Mark a missing model report as unverified.
• Add optional-family setup, dispatch rows, and lane documentation.

Verification gate

• Bun tests, strict typecheck, static invariants, and plugin validation pass.
• Install the exact candidate in Claude Code and Codex. Run keyed DeepSeek and MiniMax lanes from each real user surface, then record the installed version, action, and observed result in the PRs.
• Keep both PRs in draft until the live gate passes.

Implementation: #1 and #2.