fix: make every commandName() branch runnable when pasted

[lionello]

commandName() feeds the OAuth callback page the command a user pastes into a shell verbatim, so every branch has to be runnable on its own. The bare-basename fallback was not: for a checkout invoked from an unrelated cwd, or any install whose directory is not on the PATH, onedrive is just not a command.

Now: keep the relative path while it stays short and below the cwd, fall back to the bare name when the script's directory really is on the PATH, and use an absolute path everywhere else.

Before / after

Same function driven over a set of cwd + argv[1] + PATH combinations:

scenario	before	after
repo cwd, bin/onedrive	bin/onedrive	bin/onedrive
cwd=/, deep relative	onedrive ❌	/Users/…/bin/onedrive
global install, dir on PATH	onedrive	onedrive
sibling checkout, not on PATH	onedrive ❌	/Users/…/bin/onedrive
cwd == script dir	./onedrive	./onedrive

The ❌ rows are the bug: a bare name for something that is not on the PATH.

Checks

• process.argv[1] keeps the PATH symlink rather than resolving through to the realpath, verified with an npm-global-install-shaped symlink (bin/od -> ../lib/od.js run via PATH). So the new PATH-membership branch does fire for global installs — it is not silently dead.
• The dropped Path.isAbsolute(rel) guard is not a regression on Windows: path.win32.relative('C:\\Users\\me', 'D:\\tools\\onedrive') returns D:\tools\onedrive, which is still an absolute, runnable path whichever branch returns it.

Known rough edges (not addressed here)

• The PATH match is an exact string compare against Path.dirname(argv[1]), so a non-canonical entry (/usr/local/bin/ with a trailing slash, an unexpanded ~, or a case difference on Windows) misses and falls through to the long absolute path. Runnable, just uglier than needed — normalizing both sides would fix it.
• Paths containing spaces are returned unquoted and break when pasted (my tools/onedrive → shell reads my). Pre-existing, but the absolute-path branch makes it more reachable, and it contradicts the verbatim-runnable premise. Wants shell quoting.

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