docs: drop "forge" jargon for concrete Gitea wording

We use Gitea, not an abstract forge. Reword the docs added in this
branch: "forge thread" -> "Gitea thread", and the research note's
generic "forge" -> "Gitea" / "hosting provider" as context demands,
keeping its portability argument coherent.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

docs/decisions/README.md

@@ -3,8 +3,8 @@
 Short, durable records of decisions — one file per decision. This is a
 lightweight [Architecture Decision Record](https://adr.github.io/)
 practice: capture *what was decided and why* in a versioned file so the
-reasoning lives in the clone, not in a forge issue thread or a chat log
-that disappears when the provider does.
+reasoning lives in the clone, not in a Gitea issue thread or a chat log
+that disappears when the host does.
 
 See `docs/research/issue-tracking-vs-in-repo-decision-history.md` for
 the rationale behind keeping decision history in-repo, and
@@ -34,7 +34,7 @@ What we decided, stated plainly.
 What follows — the good, and the costs/trade-offs accepted.
 
 ## Links
-PRDs, research notes, issues/PRs. Forge links are convenience
+PRDs, research notes, issues/PRs. Gitea links are convenience
 pointers; the reasoning above must stand without them.
 ```