Slice 3 of FE-755 refactors per-slice worktree population into a hybrid
mechanism. Tracked content arrives via `git worktree add` on a slice-level
branch (`cook-slice/<runId>/<sliceId>`); untracked/gitignored content
(`node_modules/`, `dist/`, etc.) arrives via copy-on-write (`cp -c` on
macOS APFS, `cp --reflink=auto` on Linux btrfs/xfs/zfs, recursive `cpSync`
fallback).

Solves the multi-slice over-copy problem flagged as a TODO in slice 2
(~90% disk savings on CoW filesystems) while keeping runtime deps present
so pi-actions can run `npm test`/`bun test` against the slice worktree.

Slice branches use the `cook-slice/<runId>/<sliceId>` sibling namespace
rather than nesting under `cook/<runId>/<sliceId>` because git refs are
leaf-or-directory: with `cook/<runId>` already a leaf, the nested form
fails with "cannot lock ref ... 'refs/heads/cook/<runId>' exists."

OrchestratorInput.runId threads through from cook-cli for slice-branch
naming. pi-actions.ts unchanged.

Test changes:
- epic-sandbox-merge.test.ts: rewrites seedSliceFromParentWorktree tests
  around real git parents — tracked-via-git, untracked-via-CoW, slice-level
  branch checkout, sibling-slice exclusion, __epic__ exclusion.
- brownfield-smoke.integration.test.ts: passes runId; asserts the slice
  worktree is on its cook-slice/<runId>/<sliceId> branch.
- Verify: 123 test files, 1449 tests pass; 0 errors.

Also: docs/praxis/cook-brownfield.md — brunch-specific operational guide
covering pre-flight, hand-authored vs ln-scope-derived plans, the
absolute-path invocation gotcha, source-byte-identical verification,
manual artifact promotion, and cleanup.

PLAN.md updated: the multi-slice over-copy follow-on is retired (subsumed
by this work). `cook-artifact-lifecycle` remains as a separate proposed
frontier: commit slice work, replace mergeSlicesIntoEpicSandbox's file-copy
with `git merge` of slice branches (surfaces real conflicts; today's
behavior is silent last-slice-wins), and merge epic branches back to
cook/<runId> so `git merge cook/<runId>` becomes the promotion path. This
slice sets up the substrate (real slice branches) so that frontier can
land cleanly on top.

Co-Authored-By: Claude <noreply@anthropic.com>

…tion.

Renames docs/praxis/cook-brownfield.md → docs/praxis/orchestration-guide.md
to make room for the doc to grow beyond pure brownfield-mode notes (e.g.
greenfield fixture workflows, future graph-compiled plans).

Restructures the "Author the plan" section to lead with the intended
long-term target: read the plan from a spec-graph projection emitted by
`petri-graph-compilation` (currently blocked on FE-700 intent-graph-
semantics). Until that lands, two interim bridges remain — `/ln-scope`-
then-translate (most disciplined) and one-shot pi translation (cheap) —
plus hand-authoring as an escape hatch for one-off experiments.

Co-Authored-By: Claude <noreply@anthropic.com>

Fail before git worktree add when a slice id matches an existing
top-level path in the parent worktree (e.g. src/).

Co-authored-by: Cursor <cursoragent@cursor.com>

git worktree add only checks out tracked files; CoW-copy missing
top-level entries from the source cwd when creating the parent sandbox
so slice seeding inherits node_modules/dist-style artifacts. Extract
cowCopy into cow-copy.ts with complete JSDoc and reuse for slice seed.

Co-authored-by: Cursor <cursoragent@cursor.com>

When brunch cook runs against the current repo, copying .brunch would
recursively duplicate prior run worktrees into the new sandbox.

Use the exported SandboxMode alias in sandbox creation options so the type is referenced at the public API seam.

Co-authored-by: Cursor <cursoragent@cursor.com>

Codebase-mode slice worktrees carry a .git pointer file; walkFiles was
copying it into __epic__ sandboxes and giving verify-epic the wrong git
context. Skip .git/.brunch/__epic__ during merge and seed file walks.

Co-authored-by: Cursor <cursoragent@cursor.com>