Fix CLI text quoting and main-only PyPI publishing

[juanpabloguerra16]

Summary

• Fix CLI normalization for shell-stripped multi-word DSL text values, so ctxd search text:"running late" application:slack is sent to the API as text:"running late" application:slack.
• Add regression coverage for the multi-word text: CLI case.
• Change CI so PRs run tests/build only, while PyPI publishing runs only after a push to main and after the test job passes.
• Add a PyPI version existence check so publish is skipped when the current package version already exists.
• Add CONTRIBUTING.md with the package version bump and release process, and link it from README.md.

Issues

Closes #1
Closes #2
Closes #3

Tests

• env UV_CACHE_DIR=/tmp/uv-cache uv run pytest tests/test_cli.py

Files Changed

• .github/workflows/ci.yaml
• CONTRIBUTING.md
• README.md
• src/ctxd/cli.py
• tests/test_cli.py