feat(core): warm compiler caches across worktrees for compiled stacks

[OBenner]

Summary

Roadmap phase 4, second slice (follows #402's targeted test commands).

Auto Code builds every spec in a fresh worktree, so compiled projects cold-build on every coder → QA → fixer iteration. ccache/sccache keep their caches outside the worktree — pointing builds at them makes a brand-new worktree reuse object files from all previous builds.

New core/build_cache.py, wired into create_client():

Project	Env injected	Condition
CMake (C/C++)	CMAKE_C_COMPILER_LAUNCHER=ccache, CMAKE_CXX_COMPILER_LAUNCHER=ccache	ccache installed
Cargo (Rust)	RUSTC_WRAPPER=sccache	sccache installed

Safety properties:

• Never overrides user-set variables (checked against both os.environ and the collected SDK env)
• No-op when the cache tool is missing or the project is not a compiled stack
• Plain Make projects deliberately untouched — overriding CC/CXX can break builds expecting a bare compiler path
• sccache added to the rust command allowlist

Testing

tests/test_build_cache.py: 7 tests (injection per stack, missing tool, user-override respect via env dict and os.environ, non-compiled and Make-only no-ops). Ruff check + format clean; core.client import verified.

🤖 Generated with Claude Code

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Review details

⚙️ Run configuration

Configuration used: Path: .coderabbit.yaml

Review profile: ASSERTIVE

Plan: Pro

Run ID: 1c784a33-8a02-47f8-ab97-fc06613b4161

📥 Commits

Reviewing files that changed from the base of the PR and between e873cab and 12e4e13.

📒 Files selected for processing (4)

• apps/backend/core/build_cache.py
• apps/backend/core/client.py
• apps/backend/project/command_registry/languages.py
• tests/test_build_cache.py

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch feat/build-cache-env

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[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud