feat(install): add cross-platform bootstrap and end-to-end acceptance

[PsiACE]

Which issue or RFC does this PR close?

Related to #1408.

Rationale for this change

New users need installation without preinstalled uv or Python, with regional downloads and validation of the complete workflow across platforms.

What changes are included in this PR?

• Add Bash and experimental Windows PowerShell installers that reuse local tools and provision missing dependencies.
• Select download sources by explicit region, timezone, then locale. Prefer Tsinghua for PyPI in China, with USTC for uv and NJU for Python; preserve explicit source settings and fall back when automatic mirrors are unavailable.
• Organize installation methods into chapter-level tabs and document model setup in both languages.
• Extend native CI with fresh global/China installations and existing-tool reuse, then configure, start Server, remember/search, reinstall offline, and verify persistence after restart. Retain piped-terminal and Windows-output regressions.

Are there any user-facing changes?

Installers default to 1.0.0 and accept an explicit version, region, package index, and Agent integrations. Source selection applies to this installation without changing global configuration. Model credentials remain a separate setup step.

How was this change tested?

• Local Linux installation acceptance, mirror-outage fallback, region precedence, and explicit-index failure checks.
• Piped-installer regression, pre-commit checks, Bash syntax, PowerShell parsing/static analysis, workflow validation, and documentation links.
• Installation acceptance passed on Linux, macOS, and Windows. Linux native service checks passed; macOS LaunchAgent lifecycle currently fails its startup timeout, and Windows service checks are still running.

AI usage statement

Codex (GPT-6) assisted with implementation, documentation, and validation.