Add CONTRIBUTING.md + issue/PR templates

[db-tycoon-stephen]

Stand up the baseline contributor docs for external contributors:

• CONTRIBUTING.md — dev setup (pip-installable, local-first; no Docker), how to run tests (incl. the subprocess test layer), branch/release conventions, commit style.
• .github/ISSUE_TEMPLATE/ — bug report + feature request forms.
• .github/PULL_REQUEST_TEMPLATE.md.
• CODE_OF_CONDUCT.md (Contributor Covenant).

Owner: Karli (community). Part of the OSS-readiness push.

[db-tycoon-stephen]

Mirrored in Jira: PTC-25 (Epic: PTC-23 — QA & open-source community). Dev work continues here.