config: install files from the local checkout when there is one

[presto8]

get_prestobuntu_file always fetched from raw.githubusercontent.com, so
running 'bash setup' from a clone overwrote ~/.config/nvim/init.lua,
~/.gitconfig, tmux.conf and the rest with the versions published on
main -- not the ones sitting next to the script. Editing a config file
and re-running the installer silently discarded the edit, which makes
the README's 'inspect and verify the script' workflow misleading.

Prefer $PRESTOBUNTU_DIR/ when the script is running from a file,
and fall back to the download otherwise. Set PRESTOBUNTU_FORCE_DOWNLOAD=1
to keep the old always-fetch behaviour.

PRESTOBUNTU_DIR is deliberately left empty when the script is piped
into bash: BASH_SOURCE is empty in that case and 'dirname ""' yields
".", which would treat an unrelated current directory as a checkout
and install any same-named file found there. Verified the resolution
is correct both as a file and under 'cat setup | bash'.

Also makes $base local; it was leaking into the global namespace.

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🤖 Generated with Claude Code

[Copilot]

🟢 Approval recommended

The change is narrowly scoped, matches the stated problem, and preserves the existing download fallback behavior with a clear opt-out flag.

Pull request overview

This PR updates the setup installer to prefer configuration files from a local prestobuntu checkout (when the script is executed from a file) instead of always downloading from raw.githubusercontent.com, preventing local edits from being silently overwritten.

Changes:

• Detects the script’s on-disk directory via BASH_SOURCE and stores it in PRESTOBUNTU_DIR (left empty when running from stdin).
• Updates get_prestobuntu_file to install from $PRESTOBUNTU_DIR/<name> when available, with PRESTOBUNTU_FORCE_DOWNLOAD=1 to force remote downloads.
• Makes the GitHub raw URL base variable local to avoid leaking it globally.

File summaries

File	Description
setup	Prefer installing config files from a local checkout when available; add an env override to force downloads; scope URL base locally.

Review details

• Files reviewed: 1/1 changed files
• Comments generated: 0
• Review effort level: Lite

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