Route writes through the real ActiveAdmin controller, and add a create tool

[lloydwatkin]

🤖 Phase 2 of the design in #8: writes through dispatch.

What this does

create tool (new). Creates a record by dispatching the resource's own ActiveAdmin create action. Resources registered without that action are refused, permit_params decides what may be written, the namespace's authorization adapter is consulted before dispatch and again inside the controller, and every ActiveAdmin callback (before_build, before_create, before_save, …) fires.

update moved onto dispatch. It no longer calls record.update behind ActiveAdmin's back. This is a behaviour change, called out in the CHANGELOG: the controller's before_action chain, ActiveAdmin's before_update / after_update / before_save / after_save callbacks and the controller's own authorization now all run on an MCP update where they were previously skipped. Applications whose callbacks have side effects — auditing, notifications, derived columns, background jobs — will see those fire for MCP updates for the first time.

Two smaller consequences of the same change: a rejected write now reports Validation failed with the model's own messages in details, and the record echoed back has the same sensitive attributes stripped (encrypted_password, password_digest, …) that list_resources and query already omit.

ControllerDispatcher gained a block that hands the processed controller back to its caller, called whether processing succeeded or raised. That last part matters: a rejected write re-renders the admin form, and a synthesized request does not always survive that render — the unit harness proves it, blowing up on destroy_admin_user_session_path. Reading validation errors off the record rather than off the response is what keeps failure reporting honest.

The permitted-params question, resolved

The design left this open for phase 2 to decide. The from_form fallback does not earn its place, and is removed along with FormFieldCollector.

A resource that declares no permit_params cannot be saved through ActiveAdmin's own forms either: InheritedResources hands the raw unpermitted params to assign_attributes and Rails raises ActiveModel::ForbiddenAttributesError. The fallback was therefore granting MCP clients a write the admin UI itself refuses. Such a resource is now refused by name, with a message saying permit_params is missing.

A slimmed permit-params resolution survives as a pre-flight gate only — it decides whether to refuse, and reports which fields were written. The filtering itself is ActiveAdmin's job now.

FormFieldCollector is deleted rather than left as unused production code; phase 3 rewrites it substantially anyway (recording input options, not bare names) for describe_form.

Testing

• Unit: 172 examples green. record_updater_spec is replaced by record_writer_spec, running against the real ActiveAdmin harness rather than doubles — dispatch cannot honestly be tested against mocks. The harness gained permit_params and a before_save on Volunteer, plus Note (registered read-only) and Sighting (registered writable with no permit_params).
• E2E: 40 examples green. New fixture Tag resource with no permit_params, before_create / before_update callbacks on Post whose effects prove a write was dispatched rather than written straight to the model, and a title presence validation.
• Every new e2e example was mutation-checked, as CLAUDE.md requires. Two mutation runs: removing the Post callbacks and adding permit_params to Tag failed 5 examples; removing Author's actions :index, :show and Post's validation failed the other 4. Fixtures restored and both suites re-run green.

README's tool table and writes section, and the CHANGELOG, are updated.

🤖 Generated with Claude Code