[VPEX] localenv: rename compute-target vocabulary target → compute

[rugpanov]

What

Rename the compute-selection vocabulary from target to compute across the
setup-local layer, so the --output json result key, the Go model, and the
resolver names all match the invocation-side field the VS Code extension already
adopted (SetupLocalInvocation.target → compute, databricks-vscode#2039).

- "target": { "source": "serverless", "envKey": "serverless/serverless-v5", ... }
+ "compute": { "source": "serverless", "envKey": "serverless/serverless-v5", ... }

Why carry it beyond the wire key

The extension parses the CLI result with a bare structural cast, so the JSON key
must match the model field. Renaming only the tag would leave Result.Compute *TargetInfo json:"compute" — three-way naming drift for one concept. The
extension makes the same full move (PythonSetupTargetInfo →
PythonSetupComputeInfo), so this aligns the whole vocabulary.

Renamed

• Result.Target field + json tag → Compute / "compute"
• TargetInfo type → ComputeInfo
• ResolveTarget → ResolveCompute
• TargetFlags / ValidateTargetFlags → ComputeFlags / ValidateComputeFlags
• libs/localenv/target.go (+ _test) → compute.go (+ _test)

Deliberately NOT renamed

• BundleTarget — "target" is first-class bundle vocabulary (databricks.yml
targets:, --target), correctly named at that boundary.
• ErrNoTarget / E_NO_TARGET — the Go const mirrors the stable wire
  error-code string; renaming would desync it from the error-code contract.
• ComputeClient — already "compute".

schemaVersion stays 1

Not bumped: the command is still Hidden with no shipped JSON consumer, so
there's no old payload in the wild to distinguish and no compatibility window to
maintain. It will bump once the command is unveiled and the contract has real
consumers. (Confirmed with the ticket owner; drops the ticket's step-3
"tolerate both keys" work for the CLI side.)

Test

Build, unit (libs/localenv, cmd/environments), acceptance
(serverless-json/constraints-only/json-error regenerated), lint (0),
deadcode — all pass.

Part of DECO-27794 (CLI side). The extension-side model rename
(PythonSetupResult.target → compute, PythonSetupTargetInfo →
PythonSetupComputeInfo, errorMessages/fixtures/tests) is tracked separately in
databricks-vscode.

This pull request and its description were written by Isaac.

[github-actions]

Approval status: pending

/acceptance/localenv/ - needs approval

Files: acceptance/localenv/constraints-only/output.txt, acceptance/localenv/serverless-json/output.txt
Eligible: @rclarey, @anton-107, @misha-db

/cmd/environments/ - needs approval

Files: cmd/environments/compute.go, cmd/environments/sync.go
Eligible: @rclarey, @anton-107, @misha-db

/libs/localenv/ - needs approval

6 files changed
Eligible: @rclarey, @anton-107, @misha-db

_{Any maintainer (@andrewnester, @anton-107, @denik, @pietern, @shreyas-goenka, @simonfaltum, @renaudhartert-db, @janniklasrose, @lennartkats-db) can approve all areas.
See OWNERS for ownership rules.}

[eng-dev-ecosystem-bot]

Integration test report

Commit: 42f9da2

Run: 30474234806

	Env	❌​FAIL	🟨​KNOWN	💚​RECOVERED	🙈​SKIP	✅​pass	🙈​skip	Time
❌​	aws linux	3	1	3	4	319	1065	5:08
❌​	aws windows	3	1	3	4	321	1063	8:52
❌​	azure linux	3	1	3	4	319	1064	5:11
❌​	azure windows	3	1	3	4	321	1062	6:19
💚​	gcp linux			1	5	321	1066	7:46
💚​	gcp windows			1	5	323	1064	7:18

11 interesting tests: 4 SKIP, 3 FAIL, 3 RECOVERED, 1 KNOWN

	Test Name	aws linux	aws windows	azure linux	azure windows	gcp linux	gcp windows
🟨​	TestAccept	🟨​K	🟨​K	🟨​K	🟨​K	💚​R	💚​R
❌​	TestAccept/bundle/deploy/mlops-stacks	❌​F	❌​F	❌​F	❌​F	✅​p	✅​p
❌​	TestAccept/bundle/deploy/mlops-stacks/DATABRICKS_BUNDLE_ENGINE=direct	❌​F	❌​F	❌​F	❌​F	✅​p	✅​p
❌​	TestAccept/bundle/deploy/mlops-stacks/DATABRICKS_BUNDLE_ENGINE=terraform	❌​F	❌​F	❌​F	❌​F	✅​p	✅​p
🙈​	TestAccept/bundle/invariant/no_drift	🙈​S	🙈​S	🙈​S	🙈​S	🙈​S	🙈​S
🙈​	TestAccept/bundle/resources/vector_search_endpoints/drift/recreated_same_name	🙈​S	🙈​S	🙈​S	🙈​S	🙈​S	🙈​S
🙈​	TestAccept/bundle/resources/vector_search_indexes/recreate/embedding_dimension	🙈​S	🙈​S	🙈​S	🙈​S	🙈​S	🙈​S
🙈​	TestAccept/ssh/connection	🙈​S	🙈​S	🙈​S	🙈​S	🙈​S	🙈​S
💚​	TestFetchRepositoryInfoAPI_FromRepo	💚​R	💚​R	💚​R	💚​R	🙈​S	🙈​S
💚​	TestFetchRepositoryInfoAPI_FromRepo/root	💚​R	💚​R	💚​R	💚​R
💚​	TestFetchRepositoryInfoAPI_FromRepo/subdir	💚​R	💚​R	💚​R	💚​R

Top 1 slowest tests (at least 2 minutes):

duration	env	testname
5:35	gcp windows	TestAccept