build: make pyproject the single source of dependency truth

requirements.txt / requirements-dev.txt used to duplicate the dependency
pins from pyproject.toml by hand, which can silently drift. Point them at
the package itself (`.` and `-e .[dev]`) so the pins live in exactly one
place; `pip install -r requirements*.txt` keeps working.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

requirements-dev.txt

@@ -1,7 +1,4 @@
-# Development/test dependencies (mirrors [project.optional-dependencies].dev
-# in pyproject.toml).
--r requirements.txt
-pytest>=7.4
-ruff>=0.6
-mypy>=1.11
-types-requests
+# Development install. Editable package plus the [dev] extra (pytest, ruff,
+# mypy, ...). pyproject.toml remains the single source of truth; there is no
+# manual dependency mirror to keep in sync.
+-e .[dev]

requirements.txt

@@ -1,4 +1,4 @@
-# Runtime dependencies (mirrors [project.dependencies] in pyproject.toml).
-# Provided for users who want to verify/install dependencies without pip's
-# PEP 517 build isolation, e.g. `pip install -r requirements.txt`.
-requests>=2.31,<3
+# Runtime install. pyproject.toml is the single source of truth for
+# dependencies; this file just installs the package (and thus its declared
+# runtime dependencies) so `pip install -r requirements.txt` keeps working.
+.