fix(deps): drop requirements.txt, let Heroku's buildpack use uv.lock natively

heroku/heroku-buildpack-python detects the package manager by looking
for exactly one of requirements.txt, poetry.lock or uv.lock at the
repo root, and now hard-fails the build ("Multiple Python package
manager files were found") if it sees more than one — this stopped
being just a deprecation warning. Committing both uv.lock and a
uv-export-generated requirements.txt (as the previous commit did)
would have broken every Heroku deploy.

The buildpack has native uv support: it runs
`uv sync --locked --no-default-groups` against the slug's system
Python, so no requirements.txt is needed at all. Verified locally with
`UV_PROJECT_ENVIRONMENT=<tmp> uv sync --locked --no-default-groups`
(dev group correctly excluded) and
`manage.py check --settings=pythonie.settings.production` against
that environment.

Drops the now-pointless `dependencies:export` / `deps:export` tasks
and the requirements.txt mentions in CLAUDE.md/DEVELOPMENT.md, and
documents the "never commit a requirements.txt here" constraint
directly so it isn't reintroduced later. `.python-version` (already
present, Python 3.13) continues to pin the buildpack's Python version.

Caught by an independent review pass (h/t python-validator).

Author: matrixise

CLAUDE.md

@@ -125,7 +125,7 @@ task code:check
 
 ### Dependency Management
 
-Uses `uv` for fast Python package management. Dependencies are declared in `pyproject.toml` (`[project.dependencies]` + `[dependency-groups]`) and locked in `uv.lock` (committed). A `requirements.txt` is generated at the repo root purely for Heroku's buildpack — it is never edited by hand.
+Uses `uv` for fast Python package management. Dependencies are declared in `pyproject.toml` (`[project.dependencies]` + `[dependency-groups]`) and locked in `uv.lock` (committed). Heroku's Python buildpack detects `uv.lock` and installs natively via `uv sync` against the slug's system Python — **never add a `requirements.txt` at the repo root**, the buildpack aborts the build if it finds more than one package-manager file (`requirements.txt`, `poetry.lock`, `uv.lock`) at once. `.python-version` (not `runtime.txt`, which is deprecated) pins the Python version for the buildpack.
 
 ```bash
 # Recompute the lock file
@@ -149,9 +149,6 @@ task dependencies:security
 
 # Show dependencies tree
 task dependencies:tree
-
-# Regenerate requirements.txt for Heroku
-task dependencies:export
 ```
 
 ### Database Operations (Heroku)

DEVELOPMENT.md

@@ -450,9 +450,16 @@ AWS_STORAGE_BUCKET_NAME=your-bucket-name
 ```
 pyproject.toml         # [project.dependencies] + [dependency-groups] (dev)
 uv.lock                # Locked, resolved versions (committed)
-requirements.txt       # Generated via `task dependencies:export`, for Heroku only
+.python-version        # Pins the Python version for Heroku's buildpack
 ```
 
+Heroku's Python buildpack detects `uv.lock` and installs dependencies
+natively via `uv sync --locked --no-default-groups` against the slug's
+system Python — there is no separate `requirements.txt` to keep in
+sync. Do **not** add a `requirements.txt` at the repo root: the
+buildpack refuses to build if it finds more than one package-manager
+file (`requirements.txt`, `poetry.lock`, `uv.lock`) at once.
+
 **Commands**:
 ```bash
 # Recompute the lock file
@@ -475,9 +482,6 @@ task dependencies:security
 
 # Show dependencies tree
 task dependencies:tree
-
-# Regenerate requirements.txt for Heroku
-task dependencies:export
 ```
 
 **Manual process**:

Taskfile.yaml

@@ -190,11 +190,6 @@ tasks:
     cmds:
       - toast deps:tree
 
-  dependencies:export:
-    desc: Export the uv.lock to requirements.txt for Heroku deployment
-    cmds:
-      - toast deps:export
-
   docker:build:
     desc: Build the docker image
     cmds:

requirements.txt

@@ -50,14 +50,6 @@ tasks:
     cache: false
     command: uv tree
 
-  deps:export:
-    description: Export the uv.lock to requirements.txt for Heroku deployment
-    cache: false
-    mount_paths:
-      - .:/app
-    location: /app
-    command: uv export --no-dev --no-hashes --format requirements.txt -o requirements.txt
-
   code:format:
     description: Format the code
     cache: false