feat: kilnx-admin dev utility (pgAdmin-like file/data explorer)

[andreahlert]

Context

Devs working with .kilnx apps — especially those using custom fields (custom fields from "...") — need a utility to browse DB records, inspect the schema, and edit manifest files without dropping to the CLI or a raw SQLite viewer.

This is a dev utility, not a runtime app feature. Think pgAdmin: optional, not bundled into every app, for developers only.

Scope

• Browse tables and rows from the app's SQLite/Postgres database
• View and edit .kilnx files and *_fields.kilnx manifests directly (file stays source of truth)
• Apply schema migrations after manifest edits (kilnx migrate)
• Version control awareness: show git diff of edited files, let dev commit from UI
• Optional: run kilnx check inline, show diagnostics

What it is NOT

• Not an end-user UI (see companion issue for that)
• Not bundled into every compiled app binary
• Not a replacement for kilnx run/kilnx check — complements them

Possible shapes

• kilnx admin CLI command that starts a local admin server (like pgAdmin desktop mode)
• Separate kilnx-admin binary/repo
• Part of kiln-studio (already a dev-facing tool)

Relationship to custom fields

Custom fields (#58) are the primary motivator: editing *_fields.kilnx manifests, previewing field changes, viewing custom JSON column data parsed per-field.

Open questions

• Standalone binary vs sub-command of kilnx?
• Auth: localhost-only vs optional password?
• Should it support live reload of the watched .kilnx app?

[github-actions]

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