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.

src/error.rs

@@ -0,0 +1,50 @@
+//! One error type for the whole CLI.
+//!
+//! Anything that can go wrong ends up as an [`Error`], gets a `?` at the call
+//! site, and is printed once in `main` — no `unwrap` in the command paths.
+
+use std::path::PathBuf;
+
+/// Shorthand for the crate's result type.
+pub type Result<T, E = Error> = std::result::Result<T, E>;
+
+// Variants that no command constructs yet, but that the command paths below
+// are written to return. Safe to drop the allow once they are all reached.
+#[allow(dead_code)]
+#[derive(Debug, thiserror::Error)]
+pub enum Error {
+    /// Nothing at the configured path at all.
+    #[error("no kernel tree at `{path}`; set ${env} or clone one there")]
+    KernelTreeMissing { path: PathBuf, env: &'static str },
+
+    /// Something is there, but it is not a kernel source tree.
+    #[error("`{path}` is not a kernel source tree (no scripts/checkpatch.pl)")]
+    NotAKernelTree { path: PathBuf },
+
+    /// An external tool ran and failed.
+    #[error("`{program} {args}` failed (exit {code}):\n{stderr}")]
+    ExternalCommand {
+        program: String,
+        args: String,
+        code: String,
+        stderr: String,
+    },
+
+    /// An external tool could not even be started.
+    #[error("could not run `{program}`")]
+    CommandSpawn {
+        program: String,
+        #[source]
+        source: std::io::Error,
+    },
+
+    /// A patch file the user named does not exist.
+    #[error("no patch named `{name}` in `{dir}`")]
+    PatchNotFound { name: String, dir: PathBuf },
+
+    #[error(transparent)]
+    Io(#[from] std::io::Error),
+
+    #[error(transparent)]
+    Http(#[from] reqwest::Error),
+}