docs(config): add CONFIG.md and move the configuration reference out of the README

.claude/skills/update-config-docs/SKILL.md

@@ -0,0 +1,83 @@
+---
+name: update-config-docs
+description: Bring CONFIG.md back in line with the code. Use after changing anything in crates/config, the command line options, loader.conf handling, strict mode, or autoconfiguration, or when asked to check that CONFIG.md is accurate.
+---
+
+# Update CONFIG.md
+
+`CONFIG.md` is the reference for `sprout.toml`, the command line options, BLS support and strict mode.
+The README only links to it, so a setting that is missing from CONFIG.md is undocumented.
+
+## 1. Find what changed
+
+Start from the diff, or from the whole config surface if there is none:
+
+```bash
+git diff main -- crates/config crates/boot/src/options.rs crates/boot/src/config crates/boot/src/autoconfigure crates/bls
+```
+
+The sources of truth, and the section of CONFIG.md each one feeds:
+
+| Source                                                    | Section                                   |
+|-----------------------------------------------------------|-------------------------------------------|
+| `crates/config/src/lib.rs`                                | file at a glance, `options`               |
+| `crates/config/src/drivers.rs`                            | `drivers`                                 |
+| `crates/config/src/extractors*`                           | `extractors`                              |
+| `crates/config/src/actions*`                              | `actions`                                 |
+| `crates/config/src/entries.rs`                            | `entries`                                 |
+| `crates/config/src/generators*`                           | `generators`, `variants`, `exclude`       |
+| `crates/config/src/phases.rs`                             | `phases`                                  |
+| `crates/boot/src/options.rs`                              | command line options                      |
+| `crates/boot/src/main.rs`                                 | boot order, defaults, timeout and default entry order, strict mode |
+| `crates/boot/src/autoconfigure*`                          | autoconfiguration                         |
+| `crates/boot/src/generators/bls.rs`, `crates/bls`         | BLS values, boot counting, `loader.conf`  |
+
+## 2. Check each key
+
+For every struct field in `crates/config`, CONFIG.md needs its TOML name (look at `#[serde(rename)]`, as
+most multi-word keys use dashes), its type, its default, and what it does. Read the code that uses it in
+`crates/boot`, not only the doc comment, because the behavior is what the reader depends on.
+
+Look for the usual drift:
+
+- a field added, renamed or removed
+- a default that changed (`#[serde(default = ...)]`, the `Default` impls, constants such as
+  `DEFAULT_MENU_TIMEOUT_SECONDS`)
+- a new BLS value set with `context.set`, or a new option in `SproutOptions`
+- a new row to add to, or remove from, the strict mode table
+- a changed order: the default entry sources, the menu timeout sources, or the phases
+- `LATEST_VERSION` changing, which the intro and `version` text must follow
+
+Don't document what you can't confirm in the code. If a behavior is unclear, read the function that
+implements it.
+
+## 3. Write the change
+
+- Keep each section's shape: a short explanation, a TOML example, then a table of keys.
+- Examples must parse. Check that every key in one exists and is spelled the way serde expects.
+- Update the table of contents if you add or rename a heading.
+- Keep README.md short. It gets the feature list and the minimal examples, and CONFIG.md gets everything else.
+- Don't duplicate the strict mode table or the default entry order anywhere else.
+
+## 4. Write like a person
+
+Plain sentences, said once. Skip these:
+
+- filler like "robust", "seamless", "powerful", "leverage", "ensure", "note that", "it's important to"
+- groups of three added for rhythm, and a summary sentence at the end of every section
+- bullet lists for what a sentence says better, and bold used as decoration
+- em dashes (use a comma, colon or parentheses)
+
+Say what a setting does and what happens when it's unset or wrong. Name real things (`bootctl`, `PCR 12`,
+`\EFI\Linux`) instead of describing them.
+
+## 5. Verify
+
+```bash
+cargo build --target x86_64-unknown-uefi
+cargo build --target aarch64-unknown-uefi
+cargo fmt --all --check
+```
+
+Then re-read the diff of CONFIG.md against the code once more for any key whose name or default you
+touched. If the change was only to docs, skip the builds.

CLAUDE.md

@@ -2,7 +2,8 @@
 
 Sprout is a programmable UEFI bootloader written in Rust, for `x86_64` and `aarch64`. It is a `no_std` workspace
 built for the `*-unknown-uefi` targets, so it cannot be run or tested like a normal binary.
-See [README.md](README.md) for the configuration format and features.
+See [README.md](README.md) for features and [CONFIG.md](CONFIG.md) for the configuration format.
+After changing configuration, run the `update-config-docs` skill to keep CONFIG.md accurate.
 
 ## Layout