feat(devenv/pnpm): per-host shared store + hardlink import (ext4/Linux)

[schickling-assistant]

Problem

The devenv pnpm store is per-worktree (storeDir: .devenv/pnpm-store-pure-v1) with packageImportMethod: clone-or-copy. On Linux ext4 (no reflink) clone-or-copy degrades to a full copy, so every worktree/checkout of a repo duplicates its entire dependency set on disk — the dominant disk consumer on Linux dev/build hosts. On macOS APFS the same setting is a cheap native reflink, so the problem is Linux/ext4-specific.

Change

Make the pnpm store location + import method a per-host trait (gated on pkgs.stdenv.hostPlatform.isLinux), in the devenv pnpm module (nix/devenv-modules/tasks/shared/pnpm.nix):

• Linux: one shared content-addressed store at ${PNPM_SHARED_STORE_DIR:-$HOME/.local/share/pnpm/store-shared-v1} + --config.package-import-method=hardlink, so node_modules across all worktrees/sub-workspaces hardlink into a single store (marginal cost per worktree ≈ 0 for shared versions). This generalizes the existing files/-only shared-files symlink into a fully shared store (the now-redundant files-symlink is skipped on Linux).
• macOS: unchanged — keeps clone-or-copy + the per-worktree store + the shared-files symlink (native reflink is already cheap; no regression).
• CI stays job-local (the existing [ -z "$CI" ] gating is preserved — this only affects local dev/build hosts).

The committed storeDir:/packageImportMethod: policy keys are not changed; the store path + import method are driven via env/CLI config so the per-host behavior is purely additive.

Validation (representative isolated workspaces incl. a native-build dep, pnpm 11.3.0, ext4)

• node_modules are hardlinks into the store (link-count 2, shared blob inode).
• A second fresh worktree adds ~0 KB store delta (cross-worktree content shares inodes).
• Concurrent installs into the shared store are safe (serialized by the existing store-dir flock); store stays intact.
• Global virtual store single-instance / TS2742 identity holds — store location is orthogonal to the resolution model (node-linker=hoisted + global virtual store); two packages depending on the same dep resolve to one physical instance.
• Lifecycle-script packages (e.g. node-pty node-gyp rebuild) are always copy-imported even under hardlink (link-count 1) — an in-place rebuild provably cannot mutate the shared store for other worktrees. Direct mutation test confirmed. No copy-pinning needed.
• macOS trait verified unchanged via Darwin eval.

Scope / review note

This is the source-of-truth piece in the shared devenv pnpm module, opened in isolation for review. Consumers of this module pick up the per-host trait automatically; a consumer that sets PNPM_STORE_DIR itself will need a matching adjustment, which will follow separately once this is reviewed. The change is self-contained and safe to evaluate standalone.

Cutover note: store-shared-v1 is a fresh path, so the first install per host repopulates it and the old ~/.local/share/pnpm/shared-files blob can be GC'd; existing per-worktree .devenv/pnpm-store* can be cleaned to realize the disk reclaim.

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