Support directory-level caching (build artifacts and dependencies)

Problem

Builds inside the workspace are run from scratch every time, which is costly for projects with heavy or slow toolchains.

• Compiled languages such as C++ and Rust: the whole build is re-run even when most translation units are unchanged, so build intermediates are recomputed on every run.
• Interpreted languages such as Python and TypeScript: there is no compile step, but dependency resolution and installation (pip, npm/yarn/pnpm, etc.) still account for a large share of the total build time.

Proposal

Support caching of directories, such as build output directories and dependency caches, so they can be reused across runs.

Suggestions:

• Cache common directories by default, for example target/ (Rust), build/, .ccache, node_modules/, .venv/, ~/.cache/pip.
• Let users configure which paths are cached, and optionally derive the cache key from lockfiles (e.g. Cargo.lock, package-lock.json, poetry.lock).
• Provide a way to inspect and clear the cache.

Expected benefit

• Significantly shorter build times and lower CPU usage for compiled projects.
• Shorter overall build times for interpreted projects by avoiding repeated dependency downloads.

The issue names no files, tests, or entry points. Start by locating the workspace build entry point and any existing cache handling, then assess how configurable directories, lockfile-derived keys, and cache inspection or clearing fit together. Done means reusable artifact and dependency directories can be configured and managed across runs.