feat: add bootstrap release installers

[aakbarpour]

Summary

• Add secure, inspectable user-local bootstrap installers for Linux/macOS and Windows with version pinning, offline inputs, HTTPS downloads, checksum verification, and archive safety checks.
• Extend the release workflow with PR/manual dev-* builds, installer smoke tests, seven-asset checksum validation, and a tag-only publishing job with write permission.
• Update the README Quick Start and installation documentation.

Closes #57

Validation

• cargo fmt --all -- --check
• strict workspace Clippy with -D warnings
• cargo test --workspace --all-features (91 passed; one paid-provider smoke test intentionally ignored)
• cargo check --locked --workspace --all-targets
• cargo audit --deny warnings
• PowerShell parser, shell syntax, and git diff --check
• Offline installer smoke tests on Windows and simulated Unix, including a path containing spaces
• Flect blind verification/reconciliation: SAME, confidence 0.98