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/src/introduction.md

@@ -1,9 +1,9 @@
-# hotpath
+# peltier.io
 
 > Spot cooling for hot paths. A profile-guided optimization agent that
 > only ships wins it can prove.
 
-hotpath profiles real binaries and services, isolates hot paths, proposes
+Peltier profiles real binaries and services, isolates hot paths, proposes
 optimizations, and accepts a change **only** when two independent bars are
 cleared:
 
@@ -36,7 +36,7 @@ new gate.
 
 ## The differentiator
 
-Consultants sell surveys; compilers sell flags. hotpath sells **verified
+Consultants sell surveys; compilers sell flags. Peltier sells **verified
 deltas with the methodology attached** — a stopwatch on pinned hardware,
 a proof of equivalence, and an ROI figure whose confidence interval you
 can quote to a hostile reviewer. That is the entire product, and this site