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

AGENTS.md

@@ -199,7 +199,7 @@ clone, with status. Rules:
 | `docs/receipt-store.md` | Blob store + `ledger-store` SQLite persistence spec |
 | `docs/testing.md` | Every test layer, each unit's test record, and what P12 must mutate |
 | `docs/parties-and-attachments.md` | The party directory, roles, the TIN vault, attachments to any subject |
-| `bookspec/README.md` | The formal accounting specification's examples, checker and runner (P13) |
+| https://git.swaits.com/swaits/bookspec | The formal accounting specification's own repository: README, examples, checker, runner (P13); pinned in `crates/ledger-core/Cargo.toml` |
 | `deploy/README.md` | Running the books on a machine of your own (P9a) |
 | `docs/glossary.md` | Debit/credit and friends, for agents new to accounting |
 | `plan/STATUS.md` | Where the project stands right now |