feat(cli): machine-readable output for editor and automation integrations

[Caesarsage]

Give the human-facing commands a structured --output json mode so the Microcks
VS Code extension (and CI/automation) can drive the CLI without scraping text.
The rule throughout: stdout carries the structured result, stderr carries
progress and diagnostics, so a consumer can parse stdout cleanly while a human
still sees what's happening.

This is the CLI half of the VS Code integration (LFX mentorship, D3). The
extension is using the CLI as communication interface than re-implementing Microcks
auth, context, and lifecycle logic API.

Depends on

Stacked on #488 (--output framework), which is itself stacked on #491
(→ #490, error-handling refactor). This branch imports pkg/output,
pkg/errors Kind classification, and connectors.GetFullTestResult — none of
which exist on master yet — so it will not compile standalone.

Do not merge until #491 and #488 have landed. Merge order:

1. refactor(cli): migrate commands to RunE with a single exit funnel #490 → refactor(cli): remove the legacy exit shim and enforce it in CI #491 (error-handling refactor)
2. Rebase/update feat(test): add --output (text/json/yaml/github-actions) for machine-readable results #488, then merge
3. Rebase this branch onto updated master, then merge

What's in it

• capabilities command — microcks capabilities [--output json] emits a
  versioned contract (schemaVersion, cliVersion, capabilities[]) so a
  consumer can feature-detect before depending on any JSON shape.
• context --output json — list/use/delete now speak JSON; an empty config
  returns [] as a valid disconnected state.
• start --output json — prints only the structured start result on stdout;
  image-pull and readiness progress move to stderr.
• import --output json — emits an array of import results.
• test --dry-run --watch --output json — a newline-delimited JSON event
  stream (ready → imported → test-result → waiting → … → stopped), since a
  long-running watch can't wait for one final document.
• Service/test query JSON contracts (service list/get, test list/get) and
  their documentation.

Testing

• go build ./..., go vet ./..., go test ./... all pass.
• New tests cover context JSON output (list/use/empty-config) and the dry-run
  event writer (well-formed and malformed events).

Docs

documentation/cmd/{capabilities,context,import,start,test,service}.md and the
command table in README.md are updated for the new flags and JSON contracts.