fix(modules-2,6,7,8,9,10,14): deterministic main branch + correct two claims

- M2 lab now `git init -b main` (with older-git fallback note) so every later
  `main` reference resolves; reconciled M3/M6/M7/M8 wording and M10's standalone
  review-lab repo (`git init -qb main`).
- M9: replace "issues are on by default on every forge" with a provider-neutral
  version naming the exceptions (Bitbucket/Azure DevOps/SourceHut).
- M14: qualify "hosted runners need zero setup" — true for SaaS forges; the
  self-hosted track needs a runner attached (Module 19). Both paths stay valid.

Closes #5
Closes #13
Closes #16

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01TfzV5QvtPDz8LJS3Pu5VLT

modules/06-branches-sandboxes-for-experiments/README.md

@@ -47,8 +47,9 @@ Strip the mystique and a branch is **a named, movable pointer to a commit.** Tha
 definition. Your commit history is a chain of snapshots (Module 2); a branch is a sticky label that
 points at one of them and *moves forward* every time you commit on it.
 
-When you ran `git init` in Module 2, Git made one branch for you automatically — usually called
-`main`. Every commit you made moved the `main` label forward. You were "on a branch" the entire time
+When you ran `git init -b main` in Module 2, Git made one branch for you automatically — named
+`main` (the `-b main` is what guaranteed that name; in this course your repo is always on `main`).
+Every commit you made moved the `main` label forward. You were "on a branch" the entire time
 without thinking about it.
 
 The thing that surprises people coming from an ops background: **creating a branch copies nothing.**