ci(sim): keep the asset bakes cached across branches

[DavidDobas]

Follow-up to #767. #753's asset publish still re-baked the apartment (8 min) and the backrooms (12 min); two reasons, two fixes.

• Bake script split out. The CoACD step moves from build_environment_pack.py to sim/tools/decompose_pack.py, which is the only script the bake layer copies. Editing the finish step (visuals, nav map, viewer glb) no longer re-bakes a pack. build_environment_pack.py now runs the finish step alone.
• Per-branch build cache. A registry cache export replaces the ref's records, so every branch publish overwrote buildcache and evicted main's bakes. Branches now read main's buildcache plus their own cache-<branch> and write only their own. Main alone writes buildcache.
• Branch caches carry no sha- tag and are not in the keep-forever list, so cleanup-sim-images.yml ages them out after 14 days without a push.

This branch's own publish bakes the backrooms once (new bake-layer key); the apartment stays cached.

Verified: ruff, tests/ (76 passed), docker build --check -f sim/Dockerfile.assets, both tools load the backrooms scene to the same 29 parts and floor as before.

[greptile-apps]

Greptile Summary

This PR separates expensive environment-pack decomposition from the finishing stage and isolates registry build caches by branch.

• Moves CoACD decomposition into decompose_pack.py so finishing-tool edits do not invalidate the bake layer.
• Makes non-main builds read the shared main cache and a collision-resistant scoped cache while exporting only to their scoped cache.
• Keeps main as the sole writer of the shared buildcache.
• Documents branch-cache cleanup and the revised environment-pack workflow.

Confidence Score: 5/5

The PR appears safe to merge with no outstanding actionable findings.

The latest changes use PUSH_MAIN_TAGS rather than a ref-name comparison to protect the shared main cache and add a hash to sanitized non-main scopes. The only previous Greptile thread was manually resolved without explanation and therefore is not outstanding.

Important Files Changed

Filename	Overview
ci/build_assets_image.sh	Uses the authoritative main-build flag to reserve the shared cache and appends a stable hash to non-main cache scopes.
.github/workflows/publish-sim-images.yml	Supplies the GitHub ref name as the asset build's cache scope.
sim/Dockerfile.assets	Splits expensive hull decomposition into a cacheable layer independent of environment-pack finishing.
sim/tools/decompose_pack.py	Extracts the existing scene loading, floor normalization, and CoACD decomposition behavior into a dedicated tool.
sim/tools/build_environment_pack.py	Retains only the post-decomposition environment-pack finishing operations.

Flowchart

%%{init: {'theme': 'neutral'}}%%
flowchart LR
  A[Publish workflow] --> B{PUSH_MAIN_TAGS?}
  B -->|true| C[Read buildcache]
  C --> D[Write buildcache]
  B -->|false| E[Read buildcache]
  E --> F[Read cache-ref-slug-hash]
  F --> G[Write cache-ref-slug-hash]
  H[Raw environment GLB] --> I[decompose_pack.py]
  I --> J[Cached CoACD hull layer]
  J --> K[build_environment_pack.py]
  K --> L[Visual meshes, nav map, viewer assets]

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_{Reviews (2): Last reviewed commit: "ci(sim): key the shared asset cache off ..." | Re-trigger Greptile}