docs(readme): add archive notice (superseded by upstream 44983ec)

[ichoosetoaccept]

Summary

Add a CAUTION banner at the top of README.md pointing users to upstream zereight/gitlab-mcp v2.1+.

Why

Upstream commit 44983ec, released in v2.1.x, independently implements progressive disclosure and schema slimming — the two differentiators that justified this fork. Continuing to maintain a parallel implementation isn't worth the ongoing port burden (~117 upstream commits / 6 weeks at last check) given the residual differentiators (deactivation, batched activation, finer category granularity, atomic rollback) are too small to carry the maintenance cost alone.

This PR is the first step in a sunset sequence. Subsequent steps: update repo description, deprecate the npm package, close the lone open issue, cancel the Linear project, and archive the repo.

Test plan

• Pre-commit hooks pass (typos / EOF / whitespace / private-key / merge-conflict checks all green; biome + knip + Build correctly skipped — no code files changed).
• Banner placement verified: directly under H1, above the badges. Renders as a red callout in GitHub's web UI via the `> [!CAUTION]` alert syntax.
• No code paths touched — README-only change.

[ichoosetoaccept]

This change is part of the following stack:

• docs(readme): add archive notice (superseded by upstream 44983ec) #113 ◀

_{Change managed by git-spice.}

[macroscopeapp]

Approvability

Verdict: Approved

Documentation-only change adding an archive notice to README.md. No code or runtime behavior is affected.

^{You can customize Macroscope's approvability policy. Learn more.}