Adoption PR 02: first-run README and documentation guides

[aniaisec]

First-time users previously had to read benchmark tables and a long architecture/reference README before finding setup and supported entry paths. This change puts a short, no-key demonstration first and moves the complete reference material into focused guides.

• Add separate PowerShell and POSIX installation/demo commands and document current browser/desktop limits, JSON outcomes, exits, evidence and provider-key requirements.
• Add the documentation index and getting-started, own-application, configuration, CLI, integration, platform, troubleshooting, architecture, operations and extension guides. Preserve detailed evaluation, test coverage, mock target and manual review/replay material.
• Label historical reports and phase notes; correct stale surface/overlay descriptions and an invalid workflow YAML example. Keep future init/doctor/project commands clearly separate from available functionality.

Validation on 44d11f0:

• Full local suite: 975 passed, zero skips, in 22m34s, including browser, Windows desktop, demo and security cases.
• Isolated Windows desktop recheck: 15 passed in 55s with no diagnostics. The full run had emitted Windows COM 0x80010108 diagnostics but continued and exited successfully; the separate recheck was clean.
• Ruff lint/format and strict mypy for default and win32 targets passed.
• All 147 local links and JSON/YAML examples passed; historical content preservation was verified.
• Both GitHub quality and browser CI jobs passed, including deterministic benchmark and full security gates.
• Earlier clean Windows and Linux first-run demos completed all seven stages with three replays. Linux verification used a complete user-scoped Python and Chromium libraries because the minimal WSL system lacked those prerequisites; the README documents the matching venv and browser-dependency remedies.