feat: add ordered --add field mutations

[sawka-harness]

Design

When builtin set is enabled, the CLI exposes set, del, and add together without inspecting endpoint strategies or field types. A command-local pflag collector preserves every explicit operation in flag order; positional sets follow explicit flags. The field-type mutator receives the operation kind and owns its semantics: map additions insert without overwriting (same value is idempotent; a conflicting value errors), string-set additions ensure membership, and scalar additions error. The KV update path applies the same ordered map operations. Legacy SetArgs and DelArgs views remain for existing consumers. Body construction normalizes a touched field once, applies all of its operations, and encodes once before writing; an error prevents the write.

Validation

Direct mutation-body table tests cover map and set additions, existing/conflicting values, empty values, invalid operands, opposite operation orders, preserving unrelated fields, and no input mutation. A separate lifecycle test checks repeated additions across one normalization/encoding pass. Existing KV tests now include ordered mixed operations and a conflict that prevents PUT. A focused Cobra-context test checks interleaved flag capture and positional precedence. go test ./... -count=1 and task check:specs:main passed; the spec check emitted only the existing pr_label/repo_label warnings. No live API mutation was performed.

Also records the agreed test-selection guidance in AGENTS.md: test plausible non-obvious failures rather than mechanical pass-through.