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 an installation path that works without preinstalled uv or Python. Installation checks should verify the complete user workflow across supported platforms.

What changes are included in this PR?

• Add Bash and Windows PowerShell installers that reuse compatible local tools, download missing components, and respect existing index configuration.
• Document script installation, mirror selection, and model configuration in both installation guides.
• Extend Linux, macOS, and Windows service CI with acceptance tests against the current wheel: install, configure, start Server, remember/search, reinstall offline, and verify persisted data after restart.
• Keep focused regressions for interactive Agent selection through a piped installer and CLI configuration with redirected Windows output.

Are there any user-facing changes?

The installers default to 1.0.0rc2 and support an explicit release, HTTPS package index, and optional Agent integrations. Model configuration remains a separate documented step. Configuration notices use English consistently and work with redirected Windows output.

How was this change tested?

• Linux, macOS, and Windows native CI passed, including installation acceptance and service lifecycles.
• Piped-installer regression, pre-commit checks, Bash syntax, workflow validation, and documentation link checks passed.
• Related CLI and service tests: 226 passed, 1 skipped.

AI usage statement

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