Organize context fragments (#18794)

Organize context fragments under `core/context`. Implement same trait on
all of them.

codex-rs/Cargo.toml

@@ -31,7 +31,6 @@ members = [
     "core-plugins",
     "core-skills",
     "hooks",
-    "instructions",
     "secrets",
     "exec",
     "exec-server",
@@ -143,7 +142,6 @@ codex-install-context = { path = "install-context" }
 codex-file-search = { path = "file-search" }
 codex-git-utils = { path = "git-utils" }
 codex-hooks = { path = "hooks" }
-codex-instructions = { path = "instructions" }
 codex-keyring-store = { path = "keyring-store" }
 codex-linux-sandbox = { path = "linux-sandbox" }
 codex-lmstudio = { path = "lmstudio" }