* dev: document uv-based local CLI development

By default `stash` is the released uv tool install from PyPI. To develop
the CLI, provision the checkout once (`uv venv && uv pip install -e .`)
and `source .venv/bin/activate` per terminal. Standardizes all Python
installs on uv into .venv — a bare pyenv `pip install -e .` is what left
the stale `stash` binary that PR #560 had to defend against.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

* drop the never-bare-pip admonition — detection replaces memory

The shadow-install hazard is now caught mechanically (#565 warns at
session start and after stash login when duplicate stash installs are
on PATH), so the workflow doc stands as the happy path without a
remember-this rule.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

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Co-authored-by: Claude Fable 5 <noreply@anthropic.com>