docs: document complete invocation interface

[bateau84]

Purpose

Close the remaining user-facing documentation gaps before PR #45 merges.

The runner is an isolated invocation execution boundary, not an eval-suite engine. This PR makes that contract explicit and documents every supported public invocation surface.

Changes

• add docs/invocation-usage.md as the authoritative usage/interface reference;
• explicitly state that there is no input-JSON request API today;
• document the invocation input contract: CLI/Action arguments plus prompt/system/config seed files;
• document all 24/24 CLI options, including defaults and transport applicability;
• document all runner-specific environment overrides and recognized provider credentials;
• document all 21/21 GitHub Action inputs and defaults;
• document Action precedence: repository command mode, direct invocation mode, and setup-only mode;
• document which CLI capabilities are not exposed as direct Action inputs;
• add a complete "ways to run" matrix;
• clarify that eval cases/assertions/judging remain owned by the calling harness;
• correct the stale README description of the current zero-inference plugin activation preflight;
• link the complete reference prominently from README local and Action usage sections.

Validation

Cross-checked the docs against the current implementation:

• CLI flags documented: 24/24
• Action inputs documented: 21/21
• runner-specific environment overrides documented: 8/8

Documentation only; no runtime or contract behavior changes.

Targets refactor/trusted-checkout-evidence / PR #45.