Prepare ThinHarness 0.7.0

[ryanbbrown]

Summary

• Replace implicit optional capabilities with explicit plugins for filesystem tools, Bash, MCP, skills, parallel LLM calls, and subagents.
• Add narrow plugin, child-harness, lifecycle, provenance, and runtime contracts with atomic connection and cleanup behavior.
• Add provider-neutral text and image content across OpenAI, Anthropic, and OpenRouter, including image tool results and resumable transcripts.
• Split provider contracts, transports, transcript handling, and provider adapters into focused modules.
• Prepare the package and documentation for 0.7.0, remove the documentation site, and refresh the README, changelog, examples, exports, and size comparison.

This is a major pre-1.0 release with intentional breaking API changes. CHANGELOG.md contains the complete release contract.

Validation

• uv run ruff check .
• uv run pyright
• uv run pytest — 923 passed, 7 skipped, 92% coverage
• All 13 E2E journeys passed, including live OpenAI, Anthropic, OpenRouter, Langfuse, image, reasoning-resume, prompt-caching, parallel-LLM, and subagent paths
• Built and installed the wheel in a clean environment; verified public exports and provider package contents
• Completed architecture and implementation review cycles for the plugin runtime, child delegation contracts, and provider package split