JSON inspect mode: honor -tags flag (currently always emits tag)

[peczenyj]

In -inspect -format=json, the tag field on each layout field is emitted unconditionally (with omitempty only as a guard against empty strings), regardless of whether -tags was passed. This is asymmetric with text inspect mode, where -tags gates tag visibility.

Reproduction

$ ./structalign -inspect -type=Tagged ./_example/
type Tagged struct { // ...
    Flag bool    // ...                    ← no tag shown
    ID string    // ...
}

$ ./structalign -inspect -type=Tagged -tags ./_example/
type Tagged struct { // ...
    Flag bool `json:"flag"`       // ...   ← tag shown
}

$ diff \
    <(./structalign -inspect -type=Tagged -format=json ./_example/) \
    <(./structalign -inspect -type=Tagged -format=json -tags ./_example/)
# (byte-identical — both contain `"tag": "json:\"flag\""`)

Cause

internal/ui/json.go:82-91 populates Tag: f.Tag directly from the common.LayoutField. The RenderJSON method doesn't take an opt.tags parameter, and the layout phase always populates Field.Tag (internal/layout/layout.go:177).

Fix

Plumb the -tags (i.e. KeepTags) choice into RenderJSON and clear the Tag on jsonLayoutField when !keepTags, mirroring the text path. Trade-off: tooling consumers lose tag data by default — but they can opt in with -tags (or STRUCTALIGN_TAGS=true, or tags = true in .structalignrc), which is consistent with text mode.

This also means the diff-mode JSON (where original/proposed already honor -tags via align.go:90-94) becomes the natural model: -tags is the single global "include tags in output" knob, regardless of mode or format.

Scope

• Update RenderJSON signature to accept the keep-tags choice (or pass via Printer field — match existing patterns).
• Update app.go:300-307 call sites.
• Add a JSON inspect golden test case inspect_tags_tagged.json.golden with and without -tags.
• Document in README's Configuration Reference table that -tags affects both text and JSON.