chore: replace [tool.setuptools] script-files with stackvox install-helper

[StuBehan]

Summary

Drops the deprecated `[tool.setuptools] script-files` mechanism. The bash helper now ships as package data and is copied onto PATH explicitly via a new `stackvox install-helper` subcommand.

Why

`script-files` is setuptools-specific, marked as discouraged in the setuptools docs, has no PEP 621 equivalent, and doesn't round-trip through hatchling / uv. Removing it now unblocks any future build-backend migration and removes a future deprecation warning.

The naive replacement — wrap the bash helper in a Python entry point — would have added ~150–300ms of interpreter startup to every `stackvox-say` call, defeating the whole reason the helper exists (~13ms IPC round-trip from shell hooks). So instead the install step becomes explicit, opt-in, and the helper stays pure bash.

Install flow

pipx install stackvox       # `stackvox` CLI on PATH
stackvox install-helper     # copies bash helper to ~/.local/bin (opt-in)
stackvox-say "fast"         # ~13ms IPC, same as before

`--prefix DIR` overrides the destination. When the chosen prefix isn't on PATH the command prints a hint with the absolute path so users aren't left guessing why `stackvox-say` "doesn't work after install".

Changes

• Removed: `[tool.setuptools] script-files = ["bin/stackvox-say"]` and the `bin/` directory.
• Added: `stackvox/data/stackvox-say` (the same bash file, now as package data) and `stackvox/data/stackvox-say` to `[tool.setuptools.package-data]`.
• Added: `stackvox install-helper [--prefix DIR]` subcommand. Lives in `cli.py`, uses `importlib.resources.files` to locate the helper from the wheel.
• Added: `install-helper` to bash completion (with `--prefix DIR` directory completion).
• Tests: `TestCmdInstallHelper` — copy + exec bit, missing-prefix-dir creation, on-/off-PATH branches.
• Docs: README install section explains the optional helper step. Skip-if-you-don't-need-shell-hooks language so users don't think it's mandatory.

Verification

• `python -m build` produces a wheel containing `stackvox/data/stackvox-say` (verified via `unzip -l`) — no `/scripts/` entry, which is what `script-files` would have produced.
• `stackvox install-helper --prefix /tmp/test-bin` smoke-tested locally: file copies, exec bit set, off-PATH warning fires.
• 73 tests passing locally; ruff + mypy clean.

Test plan

• CI passes (lint, format, mypy, tests 3.10–3.13 Ubuntu, test-macos py3.12, PR-title, commit lint).
• On a fresh venv: `pip install dist/stackvox-*.whl && stackvox install-helper --prefix /tmp/test` produces an executable `stackvox-say`.
• CHANGELOG entry will be release-please-generated under "Chores" on the next release PR.

Migration note for users on 0.2.x

After upgrading to 0.3.0, the previously-installed `stackvox-say` (placed by setuptools' `script-files` into the venv's `bin/` dir) is no longer refreshed automatically. New installs need to run `stackvox install-helper` once. We'll call this out explicitly in the release notes.