State the required working directory explicitly in every doc

Every wrangler command in these docs silently assumed the reader's
terminal was already sitting in the cloned repo folder. Running one
from anywhere else fails with "Required Worker name missing" -- true,
but reads like a config problem, not "you're in the wrong directory."
That's exactly what happened running the Telegram webhook fix: one
command silently no-opped because of this, while the two after it
kept going with a secret that was never actually saved.

Now every doc says so upfront, and names the specific error message
to watch for.
nccommitted parent efd0f6e53f84a13a4993d0fea8c9789f084ef8ccf23a040Browse files
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