fix(build): stop piped installs from falling back to the cwd

[arelchan]

Summary

The one-line install silently installed a local checkout instead of the published release wheel.

install_raven picked its mode from dirname "$0". $0 names a real file only when
the installer runs as a file (./install.sh); piped through curl ... | sh the script
arrives on stdin, $0 is sh and its dirname is ., so the "is this a raven source
checkout?" test ran against the current working directory. Anyone running the documented
one-liner from inside a clone (a natural thing for a contributor to do) got a silent
editable install of that working tree: raven --version then reports whatever the
checkout is rather than the release, and the runtime reads a config that the checkout's
branch may not even support. The script prints Local raven source detected, but a user
who typed the official one-liner has no reason to read that as "I am not installing the
release".

Local mode now requires $0 to be an existing file, so a piped run always resolves the
release wheel. RAVEN_LOCAL_SRC=<dir> is the explicit opt-in for a piped run, and a value
that is not a raven checkout fails loudly rather than falling through to remote mode.

install.ps1 had the same fallback, through (Get-Location).Path when $PSScriptRoot is
empty under irm ... | iex; it is fixed the same way.

Type

• Fix
• Feature
• Docs
• CI / tooling
• Refactor
• Other

Verification

Ran install.sh end to end against stub uv / curl / node / npm on the PATH, so the
chosen install command is observable without touching a real environment. Four cases, all
from inside a raven checkout:

Invocation	Before	After
cat install.sh | sh	editable install of the cwd checkout	tool install --force raven[channels] @ .../raven-0.1.11-py3-none-any.whl
sh ./install.sh	editable install	editable install (unchanged)
RAVEN_LOCAL_SRC=<checkout> piped	n/a	editable install of that checkout
RAVEN_LOCAL_SRC=/nonexistent-dir piped	n/a	x RAVEN_LOCAL_SRC is not a directory: /nonexistent-dir, exit 1

• Relevant tests pass locally
• Relevant lint / type checks pass locally
• User-facing docs or screenshots are updated when needed

Notes on the unchecked box: the repo has no shell test suite and no shellcheck / shfmt hook,
so "tests" here means the stubbed end-to-end runs above; the pre-commit hooks that do exist
(ruff, prettier, eslint) do not cover .sh / .ps1. install.ps1 was reviewed by reading,
not executed, since this was verified on macOS. Both installers document RAVEN_LOCAL_SRC in
their header comment; the README one-liner is unchanged and needs no edit.

Risk

• Security impact considered
• Backward compatibility considered
• Rollback path is clear for risky changes

Notes: this narrows what a piped script will install from the machine it runs on, so it
removes a way for the installer to pick up unexpected local code. The documented developer
path (./install.sh inside a clone) is unchanged. The only behavior anyone could have relied
on is "pipe the installer while sitting in a clone and get an editable install"; that now
needs RAVEN_LOCAL_SRC=.. Rollback is a revert of this commit.

Related Issues

N/A