feat: add release installers and simplify operator documentation

[dollspace-gay]

Summary

• Replace the README's implementation inventory with short operator-focused installation, setup, task, update, and recovery instructions using ASD-STE100 writing conventions.
• Publish version-bound shell and PowerShell installers from GitHub Releases, with native x86-64 and ARM64 packages for Linux, macOS, and Windows.
• Install missing runtime dependencies through platform package tools, validate downloaded archives and installed versions, retain uninstallers, and configure the user command path.
• Offer the official Codex or Claude installer only when that account provider is selected and the user confirms; pass the discovered executable path to the daemon.
• Require all six target packages and their release evidence before publication, with regression coverage for archive rejection, dependency failures, lifecycle behavior, and release matrix completeness.

Validation

• Passed: 322 focused tests across xtask, peritus-provider-onboarding, and peritus-launcher (zero doctests).
• Passed: strict Clippy for the affected packages on Linux and the Windows GNU cross-target; formatting, ShellCheck, and Actionlint.
• Passed: cargo xtask all, including architecture, generated-asset, trust, and documentation checks.
• Passed: real Linux release package build and public bootstrap install/upgrade/uninstall smoke test in an isolated temporary user directory.
• Passed: PowerShell parser, archive, checksum, and mocked dependency tests using PowerShell 7 on Linux.

Remaining validation and release boundary

• Native macOS, Windows, and ARM CI must pass; cross-compilation and local fixtures are not native-host evidence.
• No provider installer or real provider login was run during testing.
• This PR does not publish a release. The documented public install commands become usable after the first qualified release is published.
• Runtime dependency installation does not include provider accounts, local model weights, or project-specific compilers and build tools. The installation guide states the OS prerequisites and approval requirements.