…ks#5832)

## Why

- The engine needs a real `PackageManager` implementation (uv) and a CLI
entry point so a user can actually run the feature.
- Wiring it in while keeping it `Hidden` lets the command be dogfooded
and exercised by acceptance tests without becoming user-visible until
the stack is complete.
- This is the first layer reachable from `main`, so it also makes the
whole `libs/localenv` package live for the `deadcode` checker.

## What

- **`libs/localenv/uv.go`** — the uv implementation of `PackageManager`:
discover/install uv, install the Python minor, `uv sync`, seed pip into
the venv, validate; plus the `pip.conf` → `UV_INDEX_URL` bridge for
Databricks-managed machines.
- **`cmd/localenv/`** — the command tree matching `local-env python
sync`: a `local-env` group (`Hidden: true`), a `python` subgroup, and
the `sync` verb. Parent nodes use `root.ReportUnknownSubcommand`; `sync`
uses `cobra.NoArgs`, resolves flags/bundle target, builds the `Pipeline`
with the uv manager, and renders text or `--json`. All Cobra `Use`
values + the `--json` command field come from the `libs/localenv`
constants.
- **`cmd/cmd.go`** — registers the group.

## Testing strategy

- Unit tests for uv helper logic (discovery, `pip.conf` index-url, arg
builders, stderr surfacing) (`uv_test.go`).
- Runtime smoke test of the hidden 3-level command: absent from
top-level help; help works at each level; unknown subcommand exits
non-zero; bare group shows help; flags + mutual-exclusion + `NoArgs`
behave.
- Gates: `go build ./...`, `go test`, `golangci-lint`, `deadcode` (whole
tree, no pragmas), `gofmt` — all green.
- `cmd/localenv/` unit tests are intentionally deferred to the
acceptance PR (databricks#5833), per the repo convention that user-visible CLI
output is covered by acceptance tests.

---

## About this stack

This is one of a series of small, stacked PRs that together add the
`databricks local-env python sync` command — it provisions a local
Python environment (Python version, `databricks-connect` pin, and
dependency constraints) matched to a selected Databricks compute target.
The work was split from one large branch into single-concern layers so
each is independently reviewable; the command is kept hidden until the
final PR so nothing is user-visible mid-stack.

**Review bottom-up.** Each PR targets the previous one as its base
branch (this PR targets `main`), so its diff shows only that layer.
Layers 1–5 have merged; the original layer-5 PR (databricks#5828) was split into
5a/5b/5c during review.

| # | PR | What | Status |
|---|----|------|--------|
| 1 | databricks#5823 | foundation: result types + env-key mapping | merged |
| 2 | databricks#5824 | compute-target resolution | merged |
| 3 | databricks#5826 | constraint fetch + offline cache | merged |
| 4 | databricks#5827 | formatting-preserving pyproject.toml merge | merged |
| 5a | databricks#5850 | package-manager interface + detection | merged |
| 5b | databricks#5851 | six-phase pipeline orchestrator | merged |
| 5c | databricks#5854 | --check cache purity, greenfield name, dbc insertion |
merged |
| 6 | **databricks#5832 ← you are here** | uv backend + CLI command (registered
hidden) | |
| 7 | databricks#5833 | acceptance tests | |
| 8 | databricks#5835 | unveil (unhide + help + changelog) | |

This pull request and its description were written by Isaac.

## Why

- The command's user-visible behavior — text and `--json` output, and
every error path — needs end-to-end coverage against the real CLI.
- `cmd/localenv/` carries no unit tests by design, so acceptance tests
are where that surface is verified (repo convention: user-visible CLI
output is covered by acceptance tests).

## What

- **`acceptance/localenv/`** — 9 scenarios driven through the (hidden)
command against the in-process fake server: `help` (three-level tree),
`no-target` (`E_NO_TARGET`), `flag-conflict` (Cobra mutual-exclusion),
`manager-unsupported` (conda project → clean P1 exit), `env-unsupported`
(404 → `E_ENV_UNSUPPORTED` at fetch), `json-error` (`--output json`
error object), `serverless-check` (dry-run plan), `serverless-json`
(`--json` plan), and `constraints-only`.
- Scripts use `local-env python sync` and the
`DATABRICKS_LOCALENV_CONSTRAINT_SOURCE` override; goldens show the
`local-env python sync` command field and managed marker. No source
changes.

## Testing strategy

- Goldens generated with `-update` and verified **stable on a clean
re-run** (no `-update`); all 9 subtests pass.
- `musterr` guards the five expected-failure scenarios; `trace` shows
the three output-producing ones.
- Full acceptance suite run to confirm no regressions elsewhere (only
pre-existing, environment-specific failures unrelated to this change).
- Diff confined to `acceptance/localenv/`.
- Independently verified by a review subagent (PASS — goldens, scripts,
stubs, stale-refs, hygiene) and by codex (no issues).

---

## About this stack

This is one of a series of small, stacked PRs that together add the
`databricks local-env python sync` command — it provisions a local
Python environment (Python version, `databricks-connect` pin, and
dependency constraints) matched to a selected Databricks compute target.
The work was split from one large branch into single-concern layers so
each is independently reviewable; the command is kept hidden until the
final PR so nothing is user-visible mid-stack.

**Review bottom-up.** Layers 1–5 have merged (the original layer-5 PR
databricks#5828 was split into 5a/5b/5c during review).

| # | PR | What | Status |
|---|----|------|--------|
| 1 | databricks#5823 | foundation: result types + env-key mapping | merged |
| 2 | databricks#5824 | compute-target resolution | merged |
| 3 | databricks#5826 | constraint fetch + offline cache | merged |
| 4 | databricks#5827 | formatting-preserving pyproject.toml merge | merged |
| 5a | databricks#5850 | package-manager interface + detection | merged |
| 5b | databricks#5851 | six-phase pipeline orchestrator | merged |
| 5c | databricks#5854 | --check cache purity, greenfield name, dbc insertion |
merged |
| 6 | databricks#5832 | uv backend + CLI command (registered hidden) | |
| 7 | **databricks#5833 ← you are here** | acceptance tests | |
| 8 | databricks#5835 | unveil (unhide + help + changelog) | |

This pull request and its description were written by Isaac.