feat(P1): remove gitea/gitlab from synced files + simplify docs (REQ-230,231,232)

Genericize forge-detection code: gitea→forge/generic_forge, GITEA_ACTOR→FORGE_ACTOR.
Drop .gitea byte-identity test assertions (keep GitHub-side + contract conformance).
Add test_no_forge_mentions.py guard test (REQ-230).
Delete completed migration docs (NOVA_MIGRATION.md, NOVA_AWS_MIGRATION.md).
Move NO_HUMANS_THESIS.md to .ciagent/ (internal artifact).
Strip ciagent-internal provenance from synced docs (REQ-/D-/P-/CAP- IDs,
milestone headers, .ciagent/PROJECT.md citations).
Trim README.md (reusable deploy section, local key rotation paragraph).
Fix version-tag drift (@v1.13→@v1.19, acdl/→nova/).

---ci---
project: acdl
phase: 1
milestone: v1.20
status: execute
requirements: [REQ-230, REQ-231, REQ-232]
---/ci---

docs/ONBOARDING.md

@@ -77,7 +77,7 @@ milestone (D-113).
   only (D-114); live apply is deferred.
 - **OIDC trust policy** — the onboarding Terraform uses a placeholder
   OIDC provider; real OIDC federation is blocked on
-  go-gitea/gitea#36988 (carries forward from v1.1).
+  upstream forge OIDC support (carries forward from v1.1).
 
 ## See also