Add guided first-run orderbook CLI and prepare 0.3.0

[nekrasovp]

The first-run example previously required a manually discovered Polymarket token ID. Add predxt demo for an immediate offline result and predxt explore polymarket to search by name, explicitly select a market/outcome, and display its REST orderbook. The command prints a ready-to-run WebSocket command with the selected token ID.

Changes

• Use Gamma /public-search for text search; flatten nested event markets, deduplicate IDs, preserve raw payloads, and support keyword-only active_only filtering. Nonpositive limits now raise ValueError.
• Bound each explore request, verify outcome/token and returned-book identity, keep scriptable JSON on stdout, and handle unavailable markets, invalid selection, timeout, EOF, and Ctrl-C.
• Add the first-run guide, update README/LLM context and installed-wheel CI smoke coverage, and prepare version 0.3.0 in package metadata, runtime, changelog, and lockfile. Dependency pins are unchanged.

Validation

Final head: 849ab7a70c775b91a4090f9499b27396a2b54fce; base: e2bc5ed4b73272dfa414aafb133f718176b09283.

• Fresh git clone --no-local, Python 3.12.13: Ruff, mypy, 128 tests, wheel/sdist build, Twine, strict MkDocs build, archive-content checks, and example help all passed.
• Installed 0.3.0 wheel in a separate environment; console entrypoint, version/imports, synthetic JSON, and help passed outside the repository.
• The installed-wheel live CLI fetched an explicitly selected Polymarket outcome's real orderbook without credentials. Full interactive search/selection/book flow was also verified before the metadata/docs-only release preparation.
• Negative tests cover incorrect identifiers, selection errors, HTTP 403, timeout, and cancellation with client cleanup.

Release status

This prepares 0.3.0; no release tag or PyPI publication has been created. The release workflow uses the existing pypi environment approval. explore displays one REST snapshot; continuous WebSocket operation remains a separate command.