scaffold: CLI surface with stubbed kernel and patch verbs

Add the spectral command tree -- kernel quest/test and patch
check/format/commit/create/submit/update -- with every verb wired through
main to a todo!() body, plus the pieces they will share:

- config: resolve the kernel tree from $SPECTRAL_KERNEL, falling back to
  ~/.spectral/linux, patches under ~/.spectral/patches, and confirm a tree
  really is one before anything uses it
- git: a thin wrapper over the git binary, currently only run
- error: one thiserror enum, converted once in main
- patch/checkpatch and patch/maintainers: seams around the tree's own
  scripts/checkpatch.pl and scripts/get_maintainer.pl, so spectral never
  carries a stale copy of the kernel's rules

20 functions are todo!(); the #[allow(dead_code)] sites mark the items
that are unreachable only until their caller is written. fmt, clippy -D
warnings, and test are all clean.

Cargo.lock

@@ -153,6 +153,7 @@ source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "aa8876b300ab35ba921adea3dfd70157a46249b33f95c9084ae5709785478946"
 dependencies = [
  "clap_builder",
+ "clap_derive",
 ]
 
 [[package]]
@@ -167,6 +168,18 @@ dependencies = [
  "strsim",
 ]
 
+[[package]]
+name = "clap_derive"
+version = "4.6.7"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "f9c751b79415d4e559e3d1fcf128e09e720eb673a06d26cf6f392d37d75b66e0"
+dependencies = [
+ "heck",
+ "proc-macro2",
+ "quote",
+ "syn 3.0.5",
+]
+
 [[package]]
 name = "clap_lex"
 version = "1.1.1"
@@ -475,6 +488,12 @@ version = "0.17.1"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "ed5909b6e89a2db4456e54cd5f673791d7eca6732202bbf2a9cc504fe2f9b84a"
 
+[[package]]
+name = "heck"
+version = "0.5.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "2304e00983f87ffb38b55b444b5e3b60a884b5d30c0fca7d82fe33449bbe55ea"
+
 [[package]]
 name = "html5ever"
 version = "0.39.0"
@@ -1460,6 +1479,8 @@ dependencies = [
  "clap",
  "reqwest",
  "scraper",
+ "thiserror",
+ "tokio",
 ]
 
 [[package]]
@@ -1623,9 +1644,21 @@ dependencies = [
  "mio",
  "pin-project-lite",
  "socket2",
+ "tokio-macros",
  "windows-sys 0.61.2",
 ]
 
+[[package]]
+name = "tokio-macros"
+version = "2.7.2"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "78773a2a397f451582ce068015985c33193cf6dea8b74d2a639fe457b2f07b0e"
+dependencies = [
+ "proc-macro2",
+ "quote",
+ "syn 3.0.5",
+]
+
 [[package]]
 name = "tokio-rustls"
 version = "0.26.5"