fix: use poetry self add/remove for poetry-plugin-export (#284)

## What Did You Change?

Use `poetry self add/remove` to manage `poetry-plugin-export` instead of
`pip install --user`.

## Do You Want Support (syntax, design, etc)?

N/A.

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## Any Reference Material Worth Sharing?

N/A.

---------

Co-authored-by: github-actions[bot] <github-actions[bot]@users.noreply.github.com>
Co-authored-by: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: 3 people

Makefile

@@ -7,9 +7,9 @@ DOCKER_COMPOSE_CMD := $(shell if command -v docker-compose > /dev/null; then ech
 #         of poetry.lock until it is reliable or proven useless
 .PHONY: requirements.txt
 requirements.txt: poetry.lock
-	pip install --user poetry-plugin-export \
+	poetry self add poetry-plugin-export \
 	&& poetry export -f requirements.txt --output requirements.txt \
-	&& pip uninstall --yes poetry-plugin-export
+	&& poetry self remove poetry-plugin-export
 
 
 .PHONY: build