upgrade/update: --check --json for machine-readable 'what's outdated' query

[apotema]

Context

labelle-studio (the new desktop editor) wants to surface toolchain updates without parsing human-oriented CLI output or mutating anything — see labelle-toolkit/labelle-studio#7. It needs a read-only, machine-readable "what is outdated" query for two things:

1. The CLI itself — is a newer labelle available than the installed one?
2. A project's pins — for a given project.labelle, which of its pinned packages (core/engine/gfx/assembler/plugins) have newer releases available?

Studio must notify, never auto-bump — pins are deliberate per-project choices. So this is a pure query; the existing labelle upgrade <pkg> / labelle update keep owning the actual mutation.

Proposal

Add --check --json (read-only, no writes, exit 0 even when updates exist) to the upgrade/update surface:

• labelle update --check --json → installed CLI version, latest available, update_available: bool.
• labelle upgrade --check --json (run in / pointed at a project) → per-package array:

  {
    "cli": { "installed": "1.48.0", "latest": "1.51.0", "update_available": true },
    "packages": [
      { "name": "assembler", "pinned": "0.74.2", "latest": "0.75.0", "update_available": true },
      { "name": "engine", "pinned": "1.68.0", "latest": "1.68.0", "update_available": false }
    ]
  }

Requirements

• Read-only: never edits project.labelle, never installs. --check implies dry-run.
• Stable JSON schema (versioned field or documented shape) so studio can depend on it.
• Distinguish "no newer release" from "could not reach the release endpoint" (a checked/error field per entry) so studio can show "offline" vs "up to date".
• Ideally reuse whatever release-endpoint resolution labelle update/upgrade already use, so the check and the actual upgrade agree.

Consumer

Blocks the pin-check + feature-gated version-hint slice of labelle-toolkit/labelle-studio#7 (e.g. turning a bare TemplateNotFound into "Play mode requires assembler 0.74+, this project pins 0.55.1 → Upgrade", and surfacing "assembler 0.74.2 → 0.75.0 available"). The feature-gated minimum-version hints can ship studio-side without this (studio knows its own feature min-versions); the latest-available comparison needs this query.

[apotema]

Duplicate of #276 (pre-existing). Folded the JSON-schema detail into #276; tracking there.