fix(blender-run): unique remote job dir per local path, mirrored with --delete

draupnir review: the remote job dir was keyed on the basename alone, so two
local dirs with the same name shared one remote dir, and a rerun inherited
stale files. The dir is now <basename>-<8 hex of sha256(abs path)>, and it is
mirrored with rsync --delete, confined to that one directory. There is no rm on
a computed path. Verified: a file deleted locally is gone from the rerun's
remote dir. The /defaults stderr line is documented as harmless noise.

docs/fleettools/blender.md

@@ -24,10 +24,11 @@ scripts/blender-run -- --python-expr 'import bpy; print(bpy.app.version_string)'
 ```
 
 - Always adds `-b --factory-startup --python-exit-code 1`. Arguments go after `--`.
-- **Files:** fv-ml1 does not mount `/mnt/smithy`. `--job DIR` copies DIR to
-  `fv-ml1:/tank/blender/jobs/<name>/`, runs with that as the working directory, and copies
-  new or changed files back into DIR. Nothing is deleted on either side. Use relative
-  input paths inside the job.
+- **Files:** fv-ml1 does not mount `/mnt/smithy`. `--job DIR` mirrors DIR to
+  `fv-ml1:/tank/blender/jobs/<basename>-<hash of DIR's absolute path>/`, runs with that as
+  the working directory, and copies new or changed files back into DIR. Nothing is ever
+  deleted locally, and a rerun starts from an exact mirror, never from leftovers. Use
+  relative input paths inside the job. Do not run the same DIR twice at once.
 - **Engines headless (tested 2026-09-28):** Cycles on GPU (OptiX/CUDA), Cycles on CPU, EEVEE
   (EGL, no display needed, ~9 s with the shader compile on first use), Workbench. In 5.2 the
   EEVEE id is `BLENDER_EEVEE` (`BLENDER_EEVEE_NEXT` is gone). For Cycles GPU, set
@@ -44,7 +45,9 @@ scripts/blender-run -- --python-expr 'import bpy; print(bpy.app.version_string)'
   sets it.
 - `render.filepath` must be **absolute**. Blender does not resolve a relative output path
   against the working directory ("cannot save 'out.png'"). Importers and Python file I/O do.
-- "HIPEW initialization failed" on stderr is harmless: it is the AMD backend probing.
+- Harmless stderr noise: "HIPEW initialization failed" (the AMD backend probing), and "Failed to
+  create secure directory (/defaults): Operation not permitted" (the image's runtime-dir setup
+  running as a non-root user; reported by draupnir 2026-09-28).
 - The first OptiX render in a process includes about 1-2 s of kernel load.
 
 ## blender-mcp (agents)