Improve error message for malformed hook command config

[max-sixty]

The previous error was an opaque "data did not match any variant of untagged enum CommandConfigToml" — users had no way to tell which of the three accepted forms (string, named table, pipeline list) they should use, or which value in their config was the problem.

What changed

Replace #[serde(untagged)] dispatch in CommandConfig/PipelineEntry with custom Visitor impls. Top-level type mismatches now list all three accepted forms inline with a pointer to wt hook --help; nested errors (non-string values inside a map) identify the specific offending value and point at its TOML span. Behavior for valid TOML is unchanged.

Before

TOML parse error at line 1, column 1
  |
1 | post-merge = [
  | ^
data did not match any variant of untagged enum CommandConfigToml

After

Top-level mismatch (post-merge = 42):

invalid type: integer `42`, expected a command in one of these forms:
- a string: "cargo build"
- a named table: { build = "cargo build", test = "cargo test" }
- a pipeline list: ["cargo build", { test = "cargo test" }]
run `wt hook --help` for details

Nested mismatch (post-merge = [{ build = "cargo build", ignore_exit = true }]):

TOML parse error at line 2, column 44
  |
2 |     { build = "cargo build", ignore_exit = true }
  |                                            ^^^^
invalid type: boolean `true`, expected a string

CLAUDE.md

Also adds a "Multiline String Literals" subsection to Code Quality, documenting the canonical style (plain multiline literals, no \-continuation, no concat!()) and when to use raw strings.

Testing

Three new inline-snapshot tests lock in the improved error messages (top-level wrong type, non-string in named table, non-string in pipeline map). Full suite (2962 tests + lints + doctests + doc build) passes.

This was written by Claude Code on behalf of Maximilian Roos