Organize context fragments (#18794)

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

codex-rs/Cargo.lock

@@ -1941,7 +1941,6 @@ dependencies = [
  "codex-feedback",
  "codex-git-utils",
  "codex-hooks",
- "codex-instructions",
  "codex-login",
  "codex-mcp",
  "codex-model-provider",
@@ -2074,7 +2073,6 @@ dependencies = [
  "codex-app-server-protocol",
  "codex-config",
  "codex-exec-server",
- "codex-instructions",
  "codex-login",
  "codex-otel",
  "codex-protocol",
@@ -2319,15 +2317,6 @@ dependencies = [
  "tempfile",
 ]
 
-[[package]]
-name = "codex-instructions"
-version = "0.0.0"
-dependencies = [
- "codex-protocol",
- "pretty_assertions",
- "serde",
-]
-
 [[package]]
 name = "codex-keyring-store"
 version = "0.0.0"