Consolidate contributing guide + fix test-running docs

[gbeane]

Two related developer-documentation cleanups.

1. Consolidate the contributing guide

The contributing guide existed as two divergent copies — root CONTRIBUTING.md and docs/development/contributing.md (121 lines each, with hand-adjusted and partly broken internal links). This consolidates to a single source of truth:

• Canonical file: root CONTRIBUTING.md (the conventional, GitHub-surfaced location).
• Docs site: docs/development/contributing.md is now a one-line pymdownx.snippets include (--8<-- "CONTRIBUTING.md") so the page stays in the nav with no duplicated content. (Symlinking was tried first, but zensical 0.0.34 silently skips symlinked files.)
• Fixed the broken links in CONTRIBUTING.md; cross-references now use absolute published-docs URLs so they resolve both on GitHub and in the rendered site. Fixed the two broken ../CONTRIBUTING.md links in development.md; pointed CLAUDE.md at the canonical root file. README.md already linked to root.
• Verified with zensical build (no warnings; the page renders the canonical content).

2. Fix the test-running docs (and make pytest work)

Bare pytest from the repo root failed to collect — it recursed into packages/*/tests, hitting cross-package test-module name collisions (ImportPathMismatchError) — yet the docs told contributors to run pytest.

• Added [tool.pytest.ini_options] testpaths = ["tests"] to the root pyproject.toml so bare pytest / uv run pytest runs the application suite cleanly. Explicit paths (e.g. pytest packages/jabs-core/tests) still work and override the default.
• Updated the test-running instructions in CONTRIBUTING.md and docs/development/development.md to distinguish the main suite from the per-package suites (matching the per-package list already in CLAUDE.md).
• Verified: bare pytest now collects and passes the full tests/ suite (710 tests); explicit package paths still collect.

Out of scope: docs/development/development.md also has a separate pre-existing broken link to user-guide.md (unrelated) — left for a follow-up.

[Copilot]

Pull request overview

This PR consolidates the contributing guide to a single canonical source (/CONTRIBUTING.md) and updates documentation cross-links so the contributing content stays consistent across GitHub and the rendered docs site.

Changes:

• Replaced docs/development/contributing.md with a pymdownx.snippets transclusion of the root CONTRIBUTING.md.
• Updated references in the development guide and CLAUDE.md to point at the canonical contributing guide location.
• Fixed previously broken contributing/development/user-guide links in CONTRIBUTING.md by switching to the published docs URLs.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated no comments.

File	Description
docs/development/development.md	Updates “Looking to contribute?” references to point at the published Contributing Guide page.
docs/development/contributing.md	Replaces duplicated content with a snippets include of the root CONTRIBUTING.md.
CONTRIBUTING.md	Fixes broken internal links by pointing to the published docs URLs.
CLAUDE.md	Updates the contributing-guide reference to the canonical root CONTRIBUTING.md.

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