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

[kostandinang]

Stack Context

Stacks on FE-864 (#212). 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]

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• FE-879: Lazy per-slice cook worktrees and shared node_modules for brownfield #223 👈 (View in Graphite)
• FE-864: Orchestrator improvements umbrella — brownfield feature delivery from spec #212 : 1 other dependent PR (#213 )
• FE-843: Expand cook toolchain profiles, selected at plan time #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
Behavior is intended to be correctness-neutral but changes when worktrees exist and how parallel slices share node_modules caches; collision handling on slice ids matching tracked paths is a new failure mode worth watching in brownfield runs.

Overview
Lazy slice provisioning (FE-879): Codebase mode no longer creates every slice worktree in wireHandlers before the run starts. Provisioning moves into resolveSliceCwd via new ensureSliceWorktree — first fire materializes the worktree; rework re-fires are idempotent no-ops. Runs that touch only a subset of slices pay for that many worktrees, not the full plan count.

Shared node_modules: copyMissingTopLevelEntries gains an optional symlink set; slice seeding passes SHAREABLE_TOP_LEVEL_ENTRIES (node_modules) so each slice links to the parent’s single install tree instead of a per-slice CoW copy. Other gitignored dirs (e.g. dist/) still copy per slice.

Tests / docs: New unit coverage for symlink behavior, ensureSliceWorktree idempotency and slice-id collision guards; memory/PLAN.md marks the cook over-copy follow-on resolved.

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