Neru Add Config Option

Adding a config option to Neru

Config changes fail in review when one link of the chain is missing — a default that exists on macOS but not Linux, a validator that never runs, or an example file that still shows the old key. internal/config/AGENTS.md states the contract and which test fails per link; this is the order to do it in.

Five steps are universal. Then check the conditional four — the ones that turn a one-line option into a ten-file one, and that no guide used to name.

The universal five

1. Schema. Add the struct field in internal/config/config.go with a toml:"snake_case_name" tag matching the surrounding section. Follow the existing nesting — options live in their feature's sub-struct, not at root.
2. Shared default. Set it in newDefaultConfig() in internal/config/config_defaults.go. Assign the field by name even when the value you assign is the zero value, and say in a comment what the zero means: a zero nobody wrote cannot be told from a forgotten one. TestEverySchemaFieldHasAnExplicitDefault fails on an omission.
3. Example. Add the line to configs/default-config.toml, which is the file a user copies. Comment it out if the option has no default — an uncommented empty value asserts a default that does not exist. TestConfigOptionsAppearInTheDefaultExample fails when the line is missing. Update the mode-specific examples (grid-only-config.toml, hints-only-config.toml, recursive-grid-only-config.toml) only if they show the affected section; they are deliberately partial, and only their keys are checked, by TestShippedExamplesWriteOnlySchemaKeys. Those four are the whole shipped set — configs/embed.go lists them, and anything else in configs/ is a working file rather than a project artifact. Only default-config.toml is embedded; the rest are read from disk by the tests that check them.
4. Docs. Add the row to docs/reference/configuration.md. That file is the single home for config reference facts; do not also describe the option in ARCHITECTURE or README. No test catches a missing row — this is the one universal link a reviewer has to check by eye.
5. Platform column. Add the option's path to PlatformSupport() in internal/config/platform_support.go, saying which of macOS, Linux and Windows writing it actually does something on. Say so even when the answer is all three: a column nobody wrote cannot be told from a forgotten one, and TestEveryConfigOptionDeclaresItsPlatformSupport fails on an option that is neither. A narrower column needs a note saying why, in the words the user reads — it reaches the load-time warning, the neru doctor platform_support row and the published table, which you regenerate with just gensupportref. A config.Color is declared at the field, not at its light and dark leaves (docs/adr/0013-parity-is-measured-in-words-not-subsystems.md).

Steps 2 and 3 exempt three shapes, and an option with one of them passes both tests whether or not you do the work: the light/dark leaves under a Color, a collection that ships empty, and any field of a repeated table nobody ships entries for (app_configs, layers). Do the work anyway — see internal/config/AGENTS.md.