refactor: move rule modules into src/rules/<group>/

[mgiovani]

Summary

Pure mechanical refactor for #7: the 44 flat rule files move from src/rules/<tell>.rs into src/rules/<group>/<tell>.rs, one subdirectory per groups.rs entry (artifact/, structure/, stdlib/, rhetoric/, verbosity/, sourcing/, format/). imports_data.rs stays at the top level because it is a shared table, not a rule.

Closes #7

Changes

• 44 files moved with git mv, so the diff shows renames. Each group gets a mod.rs holding only alphabetical pub mod lines, and src/rules/mod.rs shrinks to the 7 group modules plus imports_data.
• Only fully-qualified paths change: every registry::RULES entry and every cross-rule import gains a <group> segment. synonym_rotation's two super::fragmentation paths become crate::rules::rhetoric::fragmentation, because super:: now names verbosity.
• RULES order, rule codes, messages, and group names are untouched.
• AGENTS.md: "Adding a rule" steps 1 and 2 point at src/rules/<group>/, and the branch-prefix list gains refactor/, pairing with the refactor: commit type it already allowed.
• CHANGELOG.md: ### Changed entry under Unreleased.

Library path note

lib.rs declares pub mod rules, so stopslop::rules::<module> is now stopslop::rules::<group>::<module>. Nothing in tests/, bench/, or the docs used the old paths, and the stopslop::imports_data re-export is unchanged. The changelog records it; the next release should be 0.6.0.

Verification

• cargo fmt --check, cargo clippy --all-targets -- -D warnings, cargo test: all green (896 unit tests, fixture and witness suites pass).
• groups_partition_every_rule runs unchanged.
• stopslop . --list-rules and the dogfood run stopslop . (stdout and exit code) captured on main and on this branch: byte-identical.
• Grep for any remaining crate::rules::<module> without a group segment or super::fragmentation: no hits.