MegaLinter FILTER_REGEX_EXCLUDE excludes the whole .github/ tree via unanchored .git substring match

`FILTER_REGEX_EXCLUDE` in `.mega-linter.yml` is unanchored:

FILTER_REGEX_EXCLUDE: (\.git|\.venv|\.pytest_cache|\.vscode|__pycache__)

MegaLinter matches this regex against each file's full relative path. The `.git` alternative matches any path containing the substring `.git`, which also matches everything under `.github/` (e.g. `.github/workflows/mega-linter.yml`, `.github/labels.yml`, `.github/codeql.yml`). As a result the whole `.github/` tree is silently excluded from every linter — actionlint, yamllint, markdownlint, editorconfig-checker, etc. never see workflow files, label config, dependabot config, or issue templates.

Since this repo is the template other repos are scaffolded from, the bug propagates to every repo created from it.

Fix: anchor each alternative to real directory boundaries:

FILTER_REGEX_EXCLUDE: (^|/)(\.git|\.venv|\.pytest_cache|\.vscode|__pycache__)/

This keeps `.git/`, `.venv/`, `pycache/`, etc. excluded while no longer matching `.github/` as a false positive.

See laywill/laywill.github.io#48 and its fix in laywill/laywill.github.io#50 for the original writeup and verification approach (ran actionlint/yamllint/markdownlint-cli2/editorconfig-checker locally against `.github/**` via Docker to confirm no new findings before merging).

Note: fixing this may turn linters loose on `.github/**` files that have never been linted before in downstream repos — expect the fix PR to also need to clean up whatever those linters newly report.

Beginne in .mega-linter.yml und prüfe, wie FILTER_REGEX_EXCLUDE auf relative Pfade angewendet wird. Aktualisiere das Ausschlussmuster und führe anschließend actionlint, yamllint, markdownlint-cli2 und editorconfig-checker wie im Issue beschrieben über Docker für .github/** aus. Als abgeschlossen gilt die Aufgabe, wenn .github/** ohne neue Befunde gelintet wird und alle gemeldeten Probleme behoben sind.