Add keys chapter

Introduces PublicKey and SecretKey as distinct type-safe wrappers around
secp256k1, with hex and NIP-19 bech32 (npub/nsec) encoding. SecretKey has
a redacted Debug impl and no Display to reduce accidental leakage; it
exposes material only through explicit to_hex / to_nsec. FromStr on both
types auto-detects hex vs. bech32. Eight round-trip tests cover encoding,
auto-detection, prefix validation, debug redaction, and generation.

.agents/skills/write-chapter/SKILL.md

@@ -26,6 +26,10 @@ Also read:
 
 Create or update the chapter markdown file in `./book/`. Follow these conventions:
 
+- **Voice**: Write as though documenting a library, not teaching a class. Do not refer
+  to the book as a "teaching resource", "tutorial", "pedagogical", or similar — and
+  don't justify design decisions on the grounds that the reader is learning. The prose
+  should stand on its own merits as technical writing about the library.
 - **Literate style**: The prose is the primary artifact. Code blocks are woven into the
   narrative, not dumped in bulk.
 - **Code blocks** that should be tangled use the annotation format:
@@ -39,6 +43,13 @@ Create or update the chapter markdown file in `./book/`. Follow these convention
   seeing the implementation.
 - Keep code blocks focused — one concept per block where possible.
 - Ensure all `use` statements and module declarations are included in tangled blocks.
+- **Tests are hand-written, not tangled, and do not appear in the chapter.** Do not put
+  test code in chapter markdown, do not emit `{file=crate-name/tests/…}` blocks, and do
+  not add a "Tests" section to the narrative. Tangle owns `src/` and overwrites it every
+  build, so tests must live *outside* `src/` to survive. Integration tests go in
+  `crate-name/tests/<topic>.rs` as normal, hand-edited Rust files alongside `src/`. Write
+  or update the test file directly with the Edit or Write tool as part of the same task,
+  and run `cargo test -p <crate>` alongside `just all` to verify.
 - Update `./book/SUMMARY.md` if adding a new chapter.
 - Update any `Cargo.toml` files if new dependencies are needed.