refactor(sandbox): home the runtime directory under the system tmp dir

Use /tmp/fabro/runtime for both Docker and Daytona instead of
provider-specific roots. A writable /tmp inside the sandbox is already
a dependency (commit-message files, exec stop-files), it needs no
root-level mkdir for non-root container users, and it makes the two
providers uniform.

The trailing runtime path component stays load-bearing: materialized
blobs at runtime/blobs/{hash}.json are recognized as managed blob
references and normalized back to blob:// in durable context.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_012Kmn5jyrdpyCdvcfvmEDvA

lib/components/fabro-sandbox/src/daytona/mod.rs

@@ -61,7 +61,11 @@ const DAYTONA_BASH_SESSION_REMEDIATION: &str = "Daytona ran the direct command t
 
 pub(crate) const WORKING_DIRECTORY: &str = "/home/daytona/workspace";
 pub(crate) const REPOS_ROOT: &str = "/home/daytona/repos";
-pub(crate) const RUNTIME_DIRECTORY: &str = "/home/daytona/fabro/runtime";
+// Beneath the system tmp dir so any sandbox user can create it; the
+// trailing `runtime` component is load-bearing — materialized blobs at
+// `runtime/blobs/{hash}.json` are recognized as managed blob references and
+// normalized back to `blob://` in durable context.
+pub(crate) const RUNTIME_DIRECTORY: &str = "/tmp/fabro/runtime";
 const DEFAULT_SNAPSHOT: &str = "daytona-medium";
 pub const DEFAULT_DAYTONA_API_URL: &str = "https://app.daytona.io/api";
 pub(crate) const DAYTONA_DASHBOARD_SANDBOXES_URL: &str =

lib/components/fabro-sandbox/src/docker.rs

@@ -49,7 +49,11 @@ const DOCKER_BASH_REQUIREMENT: &str = "Docker sandboxes require /bin/bash for ev
 
 pub(crate) const WORKING_DIRECTORY: &str = "/workspace";
 pub(crate) const REPOS_ROOT: &str = "/repos";
-pub(crate) const RUNTIME_DIRECTORY: &str = "/fabro/runtime";
+// Beneath the system tmp dir so any container user can create it; the
+// trailing `runtime` component is load-bearing — materialized blobs at
+// `runtime/blobs/{hash}.json` are recognized as managed blob references and
+// normalized back to `blob://` in durable context.
+pub(crate) const RUNTIME_DIRECTORY: &str = "/tmp/fabro/runtime";
 const DEFAULT_GIT_CLONE_DEPTH: usize = RunCloneSettings::DEFAULT_DEPTH.unsigned_abs() as usize;
 const GIT_CLONE_TIMEOUT: Duration = Duration::from_mins(5);
 #[cfg(test)]

lib/components/fabro-workflow/src/artifact.rs

@@ -1543,12 +1543,13 @@ mod tests {
     async fn demote_materializes_remote_values_under_sandbox_runtime_directory() {
         let run_store: RunStoreHandle = make_run_store("prompt-demote-remote").await.into();
         let run_dir = tempfile::tempdir().unwrap();
-        let env = TestSyncEnv::new(false, "/workspace").with_runtime_directory("/fabro/runtime");
+        let env =
+            TestSyncEnv::new(false, "/workspace").with_runtime_directory("/tmp/fabro/runtime");
 
         let oversized = serde_json::json!("x".repeat(PROMPT_INLINE_VALUE_MAX + 1));
         let expected_bytes = serde_json::to_vec(&oversized).unwrap();
         let expected_path = format!(
-            "/fabro/runtime/blobs/{}.json",
+            "/tmp/fabro/runtime/blobs/{}.json",
             BlobHash::new(&expected_bytes)
         );
         let mut values = HashMap::from([("dataset".to_string(), oversized)]);
@@ -1605,7 +1606,8 @@ mod tests {
         let blob_hash = run_store.write_blob(&report_bytes).await.unwrap();
         let context = Context::new();
         context.set("report", fabro_types::format_blob_ref(&blob_hash).into());
-        let env = TestSyncEnv::new(false, "/workspace").with_runtime_directory("/fabro/runtime");
+        let env =
+            TestSyncEnv::new(false, "/workspace").with_runtime_directory("/tmp/fabro/runtime");
         let run_dir = tempfile::tempdir().unwrap();
 
         let resolved =
@@ -1613,7 +1615,7 @@ mod tests {
                 .await
                 .unwrap();
 
-        let expected_path = format!("/fabro/runtime/blobs/{blob_hash}.json");
+        let expected_path = format!("/tmp/fabro/runtime/blobs/{blob_hash}.json");
         assert_eq!(
             resolved["report"],
             serde_json::json!(format!("file://{expected_path}"))

lib/components/fabro-workflow/src/handler/llm/preamble.rs

@@ -788,7 +788,7 @@ mod tests {
             "security_findings",
             large_prompt_value(
                 1_843_279,
-                "/fabro/runtime/blobs/findings.json",
+                "/tmp/fabro/runtime/blobs/findings.json",
                 "{\"findings\":[\n{\"severity\":\"high\"}",
             ),
         );
@@ -798,7 +798,7 @@ mod tests {
             keys::COMMAND_OUTPUT.to_string(),
             large_prompt_value(
                 12 * 1024,
-                "/fabro/runtime/blobs/output.json",
+                "/tmp/fabro/runtime/blobs/output.json",
                 "first result\nsecond result",
             ),
         );
@@ -819,12 +819,12 @@ mod tests {
                 "\n## Completed stages\n",
                 "- **scan**: succeeded\n",
                 "  - Script: `scan --json`\n",
-                "  - Output (12.0 KB; full value: `/fabro/runtime/blobs/output.json`)\n",
+                "  - Output (12.0 KB; full value: `/tmp/fabro/runtime/blobs/output.json`)\n",
                 "    Preview: first result\n",
                 "    second result…\n",
                 "\n## Context\n",
                 "- security_findings (1.8 MB; full value: ",
-                "`/fabro/runtime/blobs/findings.json`)\n",
+                "`/tmp/fabro/runtime/blobs/findings.json`)\n",
                 "  Preview: {\"findings\":[\n",
                 "  {\"severity\":\"high\"}…\n",
             )
@@ -1703,7 +1703,7 @@ mod tests {
             "security_findings",
             large_prompt_value(
                 1_843_279,
-                "/fabro/runtime/blobs/findings.json",
+                "/tmp/fabro/runtime/blobs/findings.json",
                 "{\"findings\": [\n{\"message\": \"a | b\"}]}",
             ),
         );
@@ -1718,7 +1718,7 @@ mod tests {
 
         assert!(preamble.contains(concat!(
             "| security_findings | 1.8 MB; full value: ",
-            "`/fabro/runtime/blobs/findings.json`; Preview: ",
+            "`/tmp/fabro/runtime/blobs/findings.json`; Preview: ",
             "{\"findings\": [ {\"message\": \"a \\| b\"}]}… |",
         )));
         assert!(!preamble.contains("fabroLargeValue"));

lib/components/fabro-workflow/tests/it/integration.rs

@@ -10137,7 +10137,7 @@ impl fabro_agent::Sandbox for RemoteMockEnv {
     }
 
     fn runtime_directory(&self) -> Option<&str> {
-        Some("/fabro/runtime")
+        Some("/tmp/fabro/runtime")
     }
 
     async fn list_directory(
@@ -10445,7 +10445,7 @@ async fn downstream_remote_execution_resolves_response_blob_refs_as_text() {
     assert!(
         written
             .iter()
-            .all(|(path, _)| path.starts_with("/fabro/runtime/blobs/")),
+            .all(|(path, _)| path.starts_with("/tmp/fabro/runtime/blobs/")),
         "nothing is written outside the sandbox runtime blob directory"
     );
     assert!(