Config files open as a form in the terminal too: JSON, YAML and TOML values edited field by field #8934

Split from #7337's review (both doors). On the web, a .json, .yml/.yaml or .toml file opens as a form (#7336, #7337). In the terminal, the workspace edits the text, and the only shared part is the schema check before a commit (schemaRefusal). This was true for JSON before #7337 too.

Done when:

• [ ] The terminal has a form screen for a config file. You reach it from the workspace on a file isFormConfig accepts, with the entry where AGENTS.md says.
• [ ] Fields come from the same configFields and the same schema lookup (findSchema). Cards can be added and removed. Writing goes through writeConfig, so YAML and TOML comments survive and the read-back check holds.
• [ ] Saving commits for a writer and proposes otherwise (proposeEdit), using the web's refusals in the web's words.
• [ ] A save shows live to web viewers of the file and the branch.
• [ ] Tests cover a JSON, a YAML and a TOML edit from the terminal, each changing only the line it touched.