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/prds/README.md

@@ -1,7 +1,7 @@
 # Product requirement docs
 
 One PRD per feature: what to build, why, and how it's scoped. The PRD
-is the durable spec — it should stand on its own without a forge issue
+is the durable spec — it should stand on its own without a Gitea issue
 thread (see [`../README.md`](../README.md) for when a PRD is the right
 document vs. a research note or a decision record).
 
@@ -60,4 +60,4 @@ doesn't need (a small change rarely needs Scope or Implementation
 chunks) and add others where they help (e.g. Testing strategy,
 Alternatives considered, References). Keep the rationale self-contained
 — inline the reasoning rather than linking out to an issue thread, so
-the PRD survives a forge migration.
+the PRD survives a move off Gitea.