docs: rename the working name "hotpath" to peltier.io in README and the book

The project ships as peltier.io; "hotpath" was the placeholder working name
(CLAUDE.md: "Rename freely; nothing depends on it"). Replaces the product
name in the README front page and every mdBook source chapter — book title,
nav header, and prose ("Peltier profiles…", "Peltier sells…") — plus the
theme CSS comment.

The metaphor is unchanged (a Peltier device is a thermoelectric cooler:
"spot cooling for hot paths"), and the literal noun phrase "hot paths"
stays. Two code identifiers are intentionally left as-is because they are
not prose: the `agent/hotpath_agent/` Python module path and the
`HOTPATH_ISOLATION` env var — renaming those is a code change, not a docs
one.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01EcKPWQ3MQD4NdEHwar96u2

docs/book.toml

@@ -1,7 +1,7 @@
 [book]
-title = "hotpath"
+title = "peltier.io"
 description = "A profile-guided optimization agent that only ships wins it can prove."
-authors = ["hotpath"]
+authors = ["peltier.io"]
 language = "en"
 src = "src"