v0.3.0 release cut

[djankov]

Summary

• Bump the package version (gmat_copilot.__version__) and CITATION.cff to 0.3.0; the VS Code extension is already at 0.3.0 (lockstep).
• Aggregate the ## [0.3.0] CHANGELOG.md section: the VS Code extension (D15) and the per-model leaderboard with the never-committed held-out headline + hosted Hugging Face Space (D16).

docs/decisions.md already carries the v0.3 decisions, so the public snapshot needs no change; the README has no roadmap row to trim.

Pre-tag audit

• uv build → gmat_copilot-0.3.0-py3-none-any.whl, Requires-Python >=3.10, ships the bundled corpus + FAISS index under gmat_copilot/rag/corpus/ (corpus.jsonl, index.faiss, manifest.json). Base install stays GMAT-free.

Test plan

• CI green (test matrix 3.10/3.11/3.12 + windows, lint, typecheck, eval-smoke, minimal-install, extras-smoke, extension).
• pip install gmat-copilot==0.3.0 resolves on 3.10/3.11/3.12 with permissive base deps only (post-tag).

Release execution (post-merge, tracked in #52)

• Publish the held-out headline to the leaderboard Space via gated CI, then commit the refreshed leaderboard/leaderboard.json.
• Tag v0.3.0 → PyPI + VS Code Marketplace/Open VSX + docs site + GitHub release.
• Overwrite the GitHub release notes from the CHANGELOG.md v0.3.0 section.

Refs #52