docs: drop "forge" jargon for concrete wording

We use Gitea, not an abstract forge. Reword the pre-existing research
and PRD docs: the generic "Forge-API gate"/"forge tokens" become
"Git-host-API gate"/"Git-host tokens" (the gate still spans Gitea /
GitHub / GitLab), "Git/forge history" -> "Git/Gitea history", and the
KNOWN_FORGE_HOSTS / forge: manifest-field examples -> KNOWN_GIT_HOSTS
/ git_host:. Meaning preserved; only the word "forge" is dropped.

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

docs/research/agent-credential-proxy-landscape.md

@@ -314,9 +314,9 @@ In priority order:
    npm even if it captures something. Also disable Sentry error
    reporting via `DISABLE_ERROR_REPORTING=1`.
 
-3. **Generalize the same proxy to forge tokens.** Add a manifest
+3. **Generalize the same proxy to Git-host tokens.** Add a manifest
    field along the lines of
-   `forge: { kind: "gitea", url, tokenRef }` so a per-bottle token
+   `git_host: { kind: "gitea", url, tokenRef }` so a per-bottle token
    reference resolves at launch, the proxy starts as root before
    `node` is exec'd, and `tea` plus git HTTPS remotes are
    pre-configured to point at the proxy. Use