chore: .gitignore, .editorconfig, .dockerignore

[constk]

Problem

Without ignore-files in place from day one, generated artefacts (venvs, build outputs, node_modules, lockfile churn for transient deps) leak into commits.

Proposed solution

Port .gitignore from Teller (Python-template patterns anchored with leading / per feedback_gitignore_anchor_lib memory: /lib/, /dist/, /build/ etc. so non-Python subtrees aren't swallowed). Port .editorconfig (LF, UTF-8, 4-space Python, 2-space JS/TS/YAML). Port .dockerignore (skip .git, tests, docs from the build context).

Acceptance criteria

• .gitignore includes anchored /lib/, /dist/, /build/, __pycache__/, .venv/, node_modules/, .coverage.
• .editorconfig covers Python, JS/TS, YAML, Markdown.
• .dockerignore excludes .git, tests/, docs/, eval/, node_modules/ from build context.

Priority rationale

High: the anchoring rule isn't intuitive and the template is a teaching artefact — getting it right matters.

Depends on

#1

[constk]

Auto-close did not fire from the squash-merge body. Marking closed retrospectively — the work shipped weeks ago and the project board's Done column reflects the actual state once the project-item move that follows this comment lands.