docs(readme): add self-updating version badge, de-stale alpha prose

[piotrswierzy]

What & why

The README badge block (CI / License / Status) had no version badge — a reader couldn't tell which release OpenLinker is on. Hardcoding v0.1.0 would be the wrong fix: it'd go stale the instant the next release cuts and the README is the one place nobody remembers to update (the exact drift class we closed with the release-please work in #1332).

This adds a dynamic shields.io badge that reads the version from the single canonical source, so it can't drift:

[![Version](https://img.shields.io/github/package-json/v/openlinker-project/openlinker?label=version)](./CHANGELOG.md)

• github/package-json/v reads version from the root package.json — which release-please bumps only when a Release PR merges. So the badge tracks the release line (0.1.0 today; 0.1.1 the moment chore(main): release 0.2.0 #1345 merges) with zero README edits and no hardcoded number in the markdown.
• Chose package-json/v over github/v/release so it renders correctly right now, independent of when the v0.1.0 GitHub Release is published (the v/release variant shows "no releases" until then).

Two axes handled by churn rate:

• Version (high-churn) → dynamic badge (above).
• Maturity (low-churn) → the Status: alpha badge is left untouched — it changes ~twice in the project's life and is correctly hardcoded.

Also de-staled the prose at line 13 — dropped "First public release is in progress" (v0.1.0 is now tagged) and kept it maturity-only/version-agnostic so it can't re-drift.

Diff

README.md only, +2/−1: one new badge line + one prose trim. No app code, no tests, no migration.

Test plan

• pnpm check:invariants green — incl. scripts/check-repo-urls.mjs (the new shields URL uses the canonical openlinker-project/openlinker slug and trips no repo-URL guard).
• Status: alpha badge + its #670 link unchanged.
• No hardcoded version number in the markdown source — the number appears only inside the shields URL, resolved at render time from package.json.
• Auto-reflects a bump: the badge URL reads root package.json, which release-please updates on Release-PR merge — verify after chore(main): release 0.2.0 #1345 merges (→ 0.1.1) with no README change.

Closes #1346

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