Migrate dependency management from pip/venv to uv

[randileeharper]

Summary

Switches Vesper's dependency management and command workflow from pip/venv to uv, generating a committed uv.lock and updating all documentation and CI to use uv commands. No change to runtime behavior or dependencies.

Closes #106.

Motivation

• Reproducible environments: a committed uv.lock pins exact versions for all contributors, which pip-based editable installs do not guarantee.
• Faster, stricter resolution: uv's resolver is significantly faster and surfaces transitive conflicts that pip may resolve loosely.
• Consistency: a single tool (uv) handles environment creation, locking, syncing, and command execution.

Changes

• uv.lock — new, committed at repo root (63 packages resolved). Not gitignored.
• README.md — uv added to Requirements with install pointer; venv/pip install block replaced with uv sync --extra dev.
• docs/development.md — Environment section rewritten to use uv sync and uv run; all .venv/bin/python -m invocations replaced with uv run equivalents; stale venv prose rewritten.
• AGENTS.md — .venv/bin/python commands replaced with uv run; surrounding prose, PR-workflow line, and Notes section updated.
• .github/workflows/ci.yml — replaced pip install -e "..[dev]" with astral-sh/setup-uv@v8 + uv python install 3.12 + uv sync --extra dev; ruff/mypy/pytest now run via uv run so CI honors uv.lock.

Test commands run

uv lock --check                              # exit 0
rm -rf .venv && uv sync --extra dev          # exit 0
uv run pytest -q                             # 270 passed
uv run python -m compileall vesper tests     # exit 0
uv run ruff check vesper tests               # all checks passed
uv run mypy vesper                           # success, 0 issues in 37 files

Repo-wide grep confirms no stale pip install / venv / .venv/bin/ references remain (except the expected .gitignore heading that still ignores .venv/).

Notes

• Dev dependencies stay in [project.optional-dependencies] dev (installed via uv sync --extra dev); converting to PEP 735 dependency groups is optional future polish.