Contributor templates: issue forms, PR template, Code of Conduct, README

[karlimess]

Adds the contributor-facing templates: bug report and feature request issue forms, issue template config, PR template, Code of Conduct, and a Contributing section in the README.

Rebuilt fresh off current main rather than merging the stale #138 branch, so the history is clean and there's no README conflict.

Closes #138. Closes PTC-51, PTC-60, PTC-61, PTC-62.

Note: the failing ruff check is not from this PR. CI runs ruff unpinned (uvx ruff check), which currently resolves to 0.16.2, while the repo was last linted clean against the 0.14.0 pin in pre-commit. 0.16.2 flags import ordering across existing src/tests files, and the same failure is on main. This PR adds no Python; pytest, build, and docs all pass.

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[db-tycoon-stephen]

Approved — content is strong and the templates are ready to go. The CoC reads like a person wrote it rather than a boilerplate paste, Contributor Covenant attribution is correctly present, and the issue forms ask for the fields that actually help triage. Confirmed both labels the forms apply (bug, enhancement) exist and Discussions is enabled, so the config.yml contact link resolves.

Verified the ruff failure is pre-existing and unrelated to this PR: 79 identical violations on both this run and main's latest, zero delta. Worth noting it's not only import ordering — the mix is I001, BLE001, and RUF100, and BLE001 won't auto-fix.

Two non-blocking notes:

• README.md lost its trailing newline, which trips the end-of-file-fixer pre-commit hook — so pre-commit run --all-files fails on this branch, which is the same check the new PR template asks contributors to confirm. Worth a one-byte fixup before merge.
• Base is main rather than the active release branch (v0.1.12) that CONTRIBUTING.md prescribes. Taking it on main since these are repo-meta files, but CONTRIBUTING should get an explicit carve-out so the doc stops contradicting practice.

Two things worth tracking separately: CI runs uvx ruff check unpinned (currently 0.16.2) while pre-commit pins 0.14.0, so 'clean locally, red in CI' is now the default state. And the CoC reporting address is info@databasetycoon.com while SECURITY.md uses security@databasetycoon.com — worth confirming both are monitored.