fix: stop writing bytecode caches in installs and the test suite

[SunsetDrifter]

Summary

Install bug: on any Python that writes bytecode (any normal 3.11+), each python3 lint.py ... run created wikilint/__pycache__/ inside the wiki. The template ships no .gitignore, so the agent's next git add -A would commit .pyc files into the wiki's history. Reproduced on a field-test install: ?? wikilint/__pycache__/ after a single lint run. lint.py now sets sys.dont_write_bytecode = True before importing the engine. Recompiling about 1,900 lines per run costs milliseconds, so the cache bought nothing.

Test hygiene: the suite wrote wiki/__pycache__/lint.*.pyc into the template through py_compile in test_everything_compiles. That directory then showed up as an "unknown top-level entry" warning when linting the template. The test now uses builtin compile(), which checks syntax without writing files, and tests/helpers.py turns off bytecode writes for the whole run.

Test plan

• TDD: tests/test_no_bytecode.py written red first (a subprocess lint run in a copied install leaves no __pycache__; the suite runs with bytecode writes off; no py_compile in tests)
• Suite passes on 3.11.14 and 3.14.4 (150 tests); the template has 0 __pycache__ dirs afterwards
• Template lint back to baseline (0 errors, 2 warnings)
• Field test: fresh install, lint, then commit through the pre-commit hook leaves a clean git status and no bytecode in git ls-files