create: empty mutable-field registry breaks --set for many nouns (all AI-Evals, environment, service, repository) with no -f fallback

Summary

For many create nouns, the --set key=value flag rejects every field with unknown or read-only field "<field>", even though --help advertises those exact fields. --list-fields confirms the cause: these nouns return "No mutable fields defined for this command" — the mutable-field registry is empty, so no --set value is ever accepted. There is also no -f/--file fallback, leaving these nouns with no working create path via the CLI at all.

This appears to be the same root cause as #27 (connector) and #23 (PR), but the scope is much broader — it affects all AI-Evals nouns plus several core ones.

Version

harness version 0.1.10 (2026-06-18T04:38:52Z)

Reproduction

$ harness create eval_target --org default --project <proj> \
    --set name=repro --set identifier=repro --set type=prompt
unknown or read-only field "type"; use --list-fields to see mutable fields

$ harness create eval_target --list-fields
No mutable fields defined for this command.

--help for the same command advertises the fields it then rejects:

$ harness create eval_target --help
Create an AI Evals target: harness create eval_target --set name=... identifier=... type=prompt

Scope — --list-fields audit

Working (registry populated):

Noun	--list-fields
secret	ID / Label / Type ✅
project	ID / Label / Type ✅
organization	ID / Label / Type ✅

Broken (empty registry → --set unusable, no -f fallback):

Noun	--list-fields
eval_target	No mutable fields ❌
eval_dataset	No mutable fields ❌
eval_metric	No mutable fields ❌
eval_model	No mutable fields ❌
eval_metric_set	No mutable fields ❌
evaluation	No mutable fields ❌
pr	No mutable fields ❌ (see #23)
connector	No mutable fields ❌ (see #27)
environment	No mutable fields ❌
service	No mutable fields ❌
repository	No mutable fields ❌

No workaround

The broken nouns also reject -f:

$ harness create eval_target -f /dev/null
unknown shorthand flag: 'f' in -f

So affected resources cannot be created through the CLI by any means; callers must drop to the REST API.

Expected

Either:

1. Populate the mutable-field registry for these nouns so the --set fields shown in --help actually work (and support nested keys like git_source.file_path for git-backed eval resources), or
2. Add a -f/--file <yaml|json> body flag as a general create path for complex/nested resources.

Impact

Standing up an AI-Evals stack (dataset → target → metric set → evaluation) or any git-backed eval is impossible via the CLI today — every component create fails. Same for code pr, connector, environment, service, and repository create.

Start by tracing the create command's --set, --list-fields, and -f/--file handling, comparing the mutable-field registry for working nouns with eval_target and the other listed broken nouns. Done means the advertised fields are accepted for affected resources, including nested git_source.file_path where applicable, or a general file-based create path is available.