feat(cmd): add --json machine-readable output for all commands

[willkg]

Closes #12.

Adds a persistent --json flag that makes every subcommand (info, read, update, create, fix) emit a single structured JSON document to stdout instead of the human output, for scripting and CI. Design was worked out in _plans/015_json-output.md.

Output shape

A stable, versioned envelope for every command:

{
  "schema_version": 1,
  "markfluence_version": "1.4.0",
  "command": "update",
  "results": [ { "ok": true, "status": "published", "file": "docs/foo.md", "page_id": "123", "version": { "previous": 3, "new": 4 }, "page_width": { "value": "max", "default": false }, "attachments": [], "warnings": [], "broken": [], "error": null, "code": null } ],
  "summary": { "total": 1, "succeeded": 1, "failed": 0, "skipped": 0 }
}

• Per-command stable schema — each command always emits the same keys/shapes (empty → null/[]); the key set differs per command. schema_version bumps on breaking changes.
• Status verbs per command: published/skipped, created/not_created, changed/consistent, plus failed. info/read carry data only.
• Compound values are nested objects (version, page_width, author stamps), never display strings.
• create's two-phase abort lists every input file (failed ones with an error, the rest not_created) and sets summary.aborted: true.
• Warnings / broken image links are data (arrays on each result), not stderr log lines.

Errors & exit codes

• Per-file operational failures → results entries {ok:false, error, code}, exit 1.
• Fatal/pre-flight failures (bad flags, credential resolution) → typed error object on stderr, exit 2.
• Codes: CONFIG, AUTH, NOT_FOUND, VALIDATION, CONVERT, IO, NETWORK, API.

Implementation

• New internal/jsonout package: envelope, typed error object, code vocabulary + HTTPError mapping, shared value types.
• ui.SetJSON silences the human helpers so stdout stays pure JSON; ui.ErrSilent now carries an exit code so the root can exit 0/1/2.
• Each command's per-file worker refactored to build a typed result struct consumed by both a human renderer and the JSON collector (single source of truth).

Notable decisions made during implementation

• info/read omit a file key (they take an id/URL ARG, not a FILE) — file lives only on the batch commands.
• Exit code 2 for config/usage/bad-flag failures applies in both modes (previously such failures exited 1) — more correct, but a human-mode behavior change worth noting.

Tests

• internal/jsonout unit tests + per-command golden JSON tests built from hand-constructed result structs (no network mock needed).
• make test && make lint && make vet all green; smoke-tested end-to-end against a mock Confluence server.

[willkg]

Added a published JSON Schema for the output contract, per discussion:

• schema/json-output/v1.json (draft 2020-12) — the envelope root, per-command results/summary shapes selected via if/then on command, and the stderr error object at #/$defs/errorObject. Every object uses additionalProperties: false.
• internal/schematest — a test-only helper (santhosh-tekuri/jsonschema/v6) that compiles the schema once; each command's TestSchemaConformance validates the command's actual marshaled output against it. Because of additionalProperties: false, any new struct field fails the test until the schema is updated — so the schema can't silently drift from the implementation (verified by temporarily breaking the schema and watching the test fail).
• README links to the schema.

Note: this adds a test-only dependency (santhosh-tekuri/jsonschema/v6, plus its golang.org/x/text and dlclark/regexp2 transitives). They're reachable only from _test.go, so they're not compiled into the shipped binary — but they do land in go.mod/go.sum. Happy to drop to a lighter structural check if you'd rather not carry the dep.