Refresh README status/install copy now that v0.1.0-alpha is released

[saadqbal]

Context

v0.1.0-alpha is now published (Pre-release) and the release pipeline ran clean — signed binaries for all 5 platforms (cosign .sig/.cert), SHA256SUMS, install.sh, install.ps1. But the README still describes a pre-release world. #20 dropped Homebrew but left the release-status language untouched.

Stale lines (on develop)

• Status §: "v0.1 — feature-complete on develop; first release pending."
• "No tagged release exists yet — build from source for now."
• Customer-experience note: "install commands … light up once v0.1.0 is tagged."

Fix

Update Status + install sections to reflect the shipped v0.1.0-alpha pre-release: curl … | sh and the .ps1 path are now backed by real published, signed assets; build-from-source stays as the alt path; Homebrew stays flagged as a later follow-up; note it's an alpha/pre-release. Docs only — no behavior change.

Acceptance criteria

• No "first release pending" / "no tagged release exists yet" language remains.
• Install section reflects working install.sh/install.ps1 assets and marks the build as alpha.
• Links to the releases page where useful.

Sibling to #19/#21 (root --help refresh); separate ticket since this is release-triggered drift.