perf(build): share sccache across worktrees

[matthewgrossman]

Summary

Make the local sccache directory configurable per environment without changing
the existing CI fallback. Workstations can point SCCACHE_DIR at one shared
location, while CI can choose its own directory or remote backend.

Related Issue

None.

Changes

• Preserve a non-empty SCCACHE_DIR supplied before mise runs.
• Fall back to each worktree's existing .cache/sccache directory when the
  variable is unset, keeping current CI cache paths unchanged.
• Keep Cargo output in each worktree's local target/ directory.
• Document workstation configuration and the cross-worktree limitations of
  leaving SCCACHE_BASEDIRS unset.

Testing

• mise run pre-commit passes
• Confirmed an explicit SCCACHE_DIR reaches mise x subprocesses
  unchanged
• Confirmed an unset SCCACHE_DIR resolves to the worktree-local
  .cache/sccache fallback
• Built openshell-core from two worktrees at the same commit using an
  isolated shared sccache directory and separate Cargo target directories

Cache experiment

The experiment used sccache 0.14.0 with CARGO_INCREMENTAL=0, a fresh cache,
and a dedicated daemon. Each build used a separate Cargo target directory.

Build	Time	Cache hits	Cache misses
First worktree	1m 16s	0	200 (176 Rust, 13 C/C++, 11 assembler)
Second worktree	1m 15s	24 (13 C/C++, 11 assembler)	176 Rust

The second build had a 12% cache hit rate across cacheable compilations and no
cache read, write, or configuration errors. All Rust compilations missed because
absolute worktree paths remained part of their compiler inputs. This is why the
change makes the cache location configurable but does not set
SCCACHE_BASEDIRS or promise universal Rust reuse across worktrees.

• Unit tests added/updated (not applicable; configuration and documentation only)
• E2E tests added/updated (not applicable)

Checklist

• Follows Conventional Commits
• Commits are signed off (DCO)
• Architecture docs updated (not applicable; developer tooling behavior is documented in CONTRIBUTING.md)

The source branch name was explicitly selected for this work and does not include an issue number.

[copy-pr-bot]

Auto-sync is disabled for draft pull requests in this repository. Workflows must be run manually.

Contributors can view more details about this message here.

[github-actions]

🌿 Preview your docs: https://nvidia-preview-pr-2379.docs.buildwithfern.com/openshell