Per-video chat against the ADR-021 stored transcript, entered from the summary
view, born from observed demand (maintainer read real summaries, some made him
want to dig deeper). HARD constraint: stored-transcript-only — never fetches
captions, never touches the rate gate or YouTube, safe by construction. Default
model = the summary's model, user-switchable among the ADR-022 chain models
(doubles as model-comparison instrumentation). Ephemeral v1 (no persisted
history); chat-only/trust-the-model with show-source verification recorded as the
natural v2. ADR-027 to be appended to DECISIONS.md as the first commit.
