FE-879: Lazy per-slice cook worktrees and shared node_modules for brownfield

[kostandinang]

Stack Context

Stacks on FE-864. Refines the brownfield per-slice worktree provisioning FE-755 introduced -- makes it lazy and shares node_modules -- without changing what cook produces. (brunch serve / FE-878 is what surfaced the startup cost.)

What?

• Lazy provisioning -- slice-worktree creation moves out of the eager wireHandlers loop into resolveSliceCwd, via a new idempotent ensureSliceWorktree. A slice's worktree is materialized on first fire, so a run touching 2 of 8 slices pays for 2 worktrees, not 8. Rework re-fires are no-ops; provisioning is synchronous (execFileSync), so concurrent fires of distinct slices serialize on the JS thread -- no two git worktree add calls overlap under the parallel policy.
• Shared node_modules -- each slice symlinks node_modules/ to the parent worktree's single copy instead of CoW-copying it per slice (SHAREABLE_TOP_LEVEL_ENTRIES). walkFiles already skips symlinks, so the shared tree is never re-walked during dependency seeding, merge, or promotion. Other gitignored dirs (dist/) still copy per slice.

Why?

In codebase mode the slice layout is always per-slice, and every slice's worktree was provisioned up front -- N git worktree add + N recursive node_modules CoW copies, paid synchronously before any slice fired (9 of each for an 8-slice plan). The per-slice node_modules copy dominates wall-clock.

Behavior / risk

Correctness-neutral: same worktrees on the same branches, just lazy; deps resolve through the symlink. Trade-off: build caches under node_modules (.cache, .vite) become shared across parallel slices -- fine for cook's transient runs; documented at the call site.

Tests

npm run verify green. New unit tests: slice node_modules is a symlink (not a copy), other gitignored content still copies, ensureSliceWorktree idempotent. Brownfield integration smoke unchanged.

[kostandinang]

Warning

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Learn more

• FE-879: Lazy per-slice cook worktrees and shared node_modules for brownfield #223 👈 (View in Graphite)
• FE-864: Plan orchestrator brownfield enhancements #212 : 1 other dependent PR (#213 )
• FE-843: Toolchain profile expansion — TS runtimes + live profile selection #198
• FE-841: Embed pi as an in-process SDK to drop the external pi CLI dependency #194 : 1 other dependent PR (#211 )
• main

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PR Summary

Medium Risk
Touches brownfield sandbox layout (symlinked node_modules and shared build caches under parallel slices); behavior is intended to be correctness-neutral but parallel cook runs could see toolchain cache contention.

Overview
Brownfield codebase-mode cook no longer provisions every slice worktree at wiring time. wireHandlers drops the eager loop over all slices; resolveSliceCwd now calls idempotent ensureSliceWorktree on first use so only slices that actually fire pay for git worktree add and seeding (rework repeats are no-ops).

Slice seeding symlinks node_modules to the parent worktree via SHAREABLE_TOP_LEVEL_ENTRIES and an optional symlink set on copyMissingTopLevelEntries; other gitignored top-level dirs (e.g. dist/) still CoW-copy per slice.

memory/PLAN.md marks cook-worktree-laziness (FE-879) done and ties it to closing the over-copy follow-on from cook-codebase-mode. Tests cover symlink behavior, dist still copied, and ensureSliceWorktree idempotency.

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Cursor Bugbot has reviewed your changes and found 1 potential issue.

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Slice id collision silently ignored

High Severity

ensureSliceWorktree returns immediately when the slice path already exists, without checking that it is a provisioned git worktree. If a slice id matches a top-level tracked entry in the parent sandbox (e.g. slice src when the repo has src/), cook uses that project directory as the sandbox instead of failing, breaking per-slice isolation.

 

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