feat(yq): add jq-backed YAML processor

[chaliy]

What changed

Replaces the bespoke dotted-path yaml helper with a feature-gated yq builtin backed by the existing jq/jaq evaluator. Agents can process YAML or JSON from stdin/files with identity, nested selection, iteration, select, map, assignment, multi-document streams, YAML/JSON conversion, raw/compact/exit-status/slurp/null-input modes, combined short flags, and atomic in-place updates.

Adds parser/document/depth/output controls, deterministic bounded diagnostics, no-leak property coverage, generated builtin metadata, user docs, limitations, and threat-model entries. TOML/CSV/XML and mikefarah/yq node/style operators remain explicit unsupported boundaries.

Why

The old yaml command implemented a narrow second query language that did not match the jq-style yq surface commonly generated by agents. Sharing jq/jaq avoids duplicate evaluator semantics and inherits its work, deadline, and output controls.

Before / After

Before:

$ printf "items:\n  - type: fruit\n    name: apple\n" | yq ".items[] | select(.type == \"fruit\") | .name"
bash: yq: command not found

After:

$ printf "%s\n" "---" "type: fruit" "name: apple" "---" "type: veg" "name: kale" "---" "type: fruit" "name: pear" | yq -s -o=json -I=0 "[.[] | select(.type == \"fruit\") | .name]"
["apple","pear"]

Proof: 14/14 portable yq specs, 9 yq integration tests, arbitrary-YAML no-leak property, conditional mikefarah/yq differential, WASM feature check, real-rg regression, and post-rebase just pre-pr.

Risk

• Medium
• Removes the nonstandard yaml helper and makes yq available with the jq feature. YAML mapping keys sort deterministically at the JSON-value boundary; source order, comments, styles, and anchors are not preserved. Custom tags and non-string keys fail closed. In-place writes depend on VFS sibling rename semantics and are covered for success, parse/filter/exit-status failure, stdout suppression, and mode preservation.

Checklist

• Tests added or updated
• Backward compatibility considered

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