Read Rails YAML by its root key in check without a config

[arvida]

Fixes localheroai/localhero-ai#803.

Rails loads a YAML file as the locale its root key names, whatever the file is called. check took the locale from the file name, so config/locales/sr.yml starting with sr-Latn: had every key reported as an orphan (sr-Latn.hello) and every source key as missing. On a clean run over chatwoot/chatwoot that was 986 false orphans, all from sr.yml, devise.sr.yml and ur_IN.yml (root ur:), and the summary line a first-time user reads was wrong.

Without a localhero.json, check now reads such a file under its file's locale and says so once:

ℹ config/locales/sr.yml is named for sr but defines sr-Latn, the locale Rails loads it as. Checked it as sr.

chatwoot	before	after
summary	85% complete, 986 orphans	88% complete, 0 orphans
sr	5%, 518 orphans	88%, 0 orphans
ur_IN	13%, 468 orphans	88%, 0 orphans

Scope

• Only check, and only without a localhero.json. With a config, check keeps reading files the way translate does. Shared loaders (files.ts, translation-utils.ts) are untouched; isLanguage is exported from locale-detection.ts, nothing else changes there.
• Only when every source YAML file is wrapped in the source locale, so plain YAML whose single namespace happens to be a language code (id:) is left alone.
• A target whose root key is the source locale (de.yml starting with en:) is still reported. Rails would load it as the source language.
• The tree is re-serialised under the file's locale rather than renamed in the text, so any valid YAML spelling of the root key works.
• --changed-only aligns base files too, judged by the base's own source files, so a key dropped in a pull request is still flagged as new.

Known limits

• In --format github, findings in a realigned file come without a line number (the line lookup reads the original file). Before this they were all false orphans.
• The info line is text output only.
• ur_IN.yml is checked as ur_IN, not merged into ur as Rails does. Pairing files to sources stays filename-based.

Verified

• Full suite passes, lint and build clean. Tests cover the plain and "sr-Latn" : root spellings, a comment-only file, a source-locale root, an unwrapped id: namespace, a key dropped under --changed-only, and a base branch whose source was not yet wrapped.
• Built CLI on chatwoot as above; lago-api and excalidraw (--path packages/excalidraw/locales) give the same summary as 0.0.76.