Fix missed quiet "yes" after phone confirmation: more sensitive VAD

After the phone confirmation a caller's "yes" wasn't picked up (silence) until
they repeated it louder. Logs: line was live and the half-duplex gate had
reopened, but VAD never fired for ~14s — the quick/quiet "yes" was below
threshold (min_volume 0.3, start_secs 0.2).

Now that HalfDuplexGate gates out the agent's echo while it speaks, VAD can be
sensitive without echo false-triggers (it only listens hard on the caller's
turn). Lowered min_volume 0.3->0.15, start_secs 0.2->0.1, and trimmed the echo
tail 0.5->0.25 so an answer right after the agent stops isn't dropped.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

.env.example

@@ -62,4 +62,9 @@ HANGUP_DELAY_SECS=4.0
 # Half-duplex: ignore caller audio while the agent speaks (+ tail) so its own echo on the
 # phone line can't trigger a false barge-in that cancels its reply. false = allow barge-in.
 HALF_DUPLEX=true
-ECHO_TAIL_SECS=0.5
+ECHO_TAIL_SECS=0.25
+# VAD kept sensitive (half-duplex gates echo, so this only affects the caller's turn).
+VAD_CONFIDENCE=0.5
+VAD_MIN_VOLUME=0.15
+VAD_START_SECS=0.1
+VAD_STOP_SECS=0.5