notebooklm: send audio custom instructions as UniversalAudioDetails.prompt

[faughnn]

Problem

Custom instructions for an Audio Overview are currently unusable. Any non-empty
instruction string fails:

nlm audio create <notebook> "Educational walkthrough; spell out acronyms."
→ InvalidInput (API error 3 on R7cb6c)

The same notebook succeeds immediately with "", so it is not an auth, source
or notebook problem.

Cause

CreateAudioOverviewWithOptions gates the CreateUniversalArtifact path on
opts.Instructions == "":

if opts.Instructions == "" && opts.AudioType == DEEP_DIVE &&
   opts.Length == DEFAULT && opts.Language == "en" {
        → CreateUniversalArtifact          // accepted by the server
}
// otherwise
req := &pb.CreateAudioOverviewRequest{ CustomInstructions: opts.Instructions, ... }
        → CreateAudioOverview              // legacy; server rejects

So supplying instructions actively diverts the request away from the shape the
server accepts and into the legacy one it no longer does. Meanwhile
UniversalAudioDetails already carries a prompt field (field 1) intended for
exactly this, and nothing populates it.

Fix

Populate prompt from opts.Instructions and drop the emptiness condition from
the gate, so instructions ride the modern request rather than diverting to the
legacy one. Behaviour with empty instructions is unchanged.

Verification

Against the live API, not just unit tests:

• Before: any non-empty instruction returned InvalidInput.
• After: the request is accepted, and the instruction is honoured. An overview
  told to open with a specific marker phrase began with exactly that phrase.

One caveat worth recording for anyone testing this: a sweeping style instruction
("speak entirely in limericks") was accepted but not followed, so the model
appears to decline wholesale format overrides. Targeted instructions about
content, emphasis and delivery do take effect. Absence of an error is therefore
not proof the steer applied — check the output.

Test

Adds TestUniversalAudioDetailsPromptRoundTrip, pinning prompt at wire index
0. A regression there would silently drop the steer and generate default audio
with no error surfaced, which is the failure mode this change exists to remove.

go test ./notebooklm/... ./cmd/... passes; no existing test asserted the old
gating.