docs: comprehensive project README

[Edcko]

Summary

Rewrites the README to accurately reflect the current state of the project, including all features added since the initial version.

What changed

• Added Gemini provider — config, setup instructions, and environment variable documentation
• Updated tools section — 11 tools (was 6): added list_dir, git, web_fetch, delegate, subagent
• Updated skills section — 8 built-in skills (was 3): added python, react, docker, database, api_design
• Added sub-agent architecture docs — researcher, coder, reviewer, tester agents with parallel delegation
• Added installation methods — Homebrew tap, curl installer, go install, build from source
• Added techne doctor and techne config init/show — CLI commands added in recent commits
• Added slash commands section — /model, /provider, /clear, /help with keyboard shortcuts
• Added TUI features — markdown rendering, syntax highlighting, diff display, status bar, multiline input
• Added context management — token tracking, automatic compression, LLM-powered summarization
• Added E2E test docs — real Ollama integration test suite
• Updated architecture tree — reflects all current directories including e2e/, Formula/, tui/diff/, tui/markdown/
• Centered header with badges, quick navigation links, and cleaner visual hierarchy
• Updated test coverage table — covers all test packages with actual test counts from commits

Before / After

• Before: 532 lines, outdated feature list (6 tools, 3 skills, 3 providers)
• After: 479 lines, comprehensive and accurate (11 tools, 8 skills, 4 providers)