install: simplify cross-platform setup

[lbx154]

What changed

• Replace mixed installation instructions with explicit Windows/macOS/Linux contracts: direct pip on Windows without a venv, uv tool on macOS, and a persistent source venv on Linux.
• Require setup to complete backend readiness plus a real Agent CLI turn before persisting or reporting success.
• Preserve the backend model-selector work from fix(setup): configure and validate the backend model selector #44 while carrying backend-native defaults through Manager, per-role overrides, daemon handoff, and Doctor; non-OpenAI backends are not given an OpenAI model id unless explicitly configured.
• Add Code-Agent-assisted Doctor analysis over sanitized deterministic findings. Claude/Copilot/OpenCode/Pi/Grok advisors run with tools disabled; Codex fails closed because its CLI has no tool-free prompt mode.
• Improve Windows remediation and claude.cmd stdin transport, and treat Node as bundled/not required for frozen Desktop diagnostics.
• Add wheel/sdist and Windows installer/portable release jobs with tag/version validation and least-privilege publish permissions.
• Add Windows direct-pip and macOS uv tool CI smokes plus English/Chinese installation documentation.

Public-main integration

This branch is rebased on current public main e8b2e7cd and preserves the newly merged Windows docs (#43), Qoder/dsh support, and backend model-selector behavior (#44).

Validation

• Public targeted setup/backend/Doctor/model/Windows/install suite passed (187 tests), followed by release artifact rebuild and release identity tests.
• Private release-candidate validation passed 5,109 tests with 24 skips after excluding three failures independently reproduced on its baseline; the one later regression was fixed and rerun.
• Clean macOS uv tool wheel installation passed with real Pi 0.84.1 setup and tool-free Doctor analysis; no private path appeared in advisor output.
• Clean B200 Linux wheel installation passed; durable direct submission returned in 0.143s and completed independently.
• Ruff, focused Mypy, workflow YAML parsing, release checks, and git diff --check pass.

Release prerequisites

PyPI Trusted Publishing and Windows code signing still require repository/release configuration outside this PR.