refactor(bookspec): the specification lives in its own repository (P13 step 6)

bookspec/ (spec, examples, checker, runner, Lean, GnuCash adapter) moves to
https://git.swaits.com/swaits/bookspec, history kept by git subtree split
(public home; CC BY 4.0 for spec/ and examples/, MIT OR Apache-2.0 for the
code). ledger-core and ledger-package depend on bookspec-checker and
bookspec-runner as git dev-dependencies pinned to
74f2853d5f0db0066147c17fed8033cfe3d83a24, and read the examples through
bookspec_checker::examples(). just check still runs every example via
cargo test --workspace. The pin check against docs/spec/sources and the
rules.md id check move into tests/bookspec.rs (they need this repository's
files). The accounting is unchanged: no example or rule moved.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01HS8WZziDnyxunC23MUxrLp

.gitignore

@@ -7,4 +7,3 @@ site/dist/
 /mutants.out*/
 /mutants-*/
 # bookspec's own workspace target
-/bookspec/target/