docs: fix genuine passive-voice hits in docs/web-ui (small files)

agent.md, css-vars.md, design-guide.md, shape.md, terminal-rendering.md --
26 of 40 write-good.Passive hits rewritten to name the actor (hive-agent,
browsers, the harness, the client, lifecycle::{spawn,rebuild}, CSS, the
router, ...), reusing an actor already named nearby wherever one exists.
14 left alone: established config-state conditionals ("when X is
set/enabled/given/called"), negative-capability/state-descriptor idioms
("is gone", "is hidden", "is unchanged"), one false-positive
("typed slash commands" as a compound noun, not a passive action), a
backward-looking changelog fact with no actor worth naming, and two
deliberately-parallel rhetorical contrasts (sanitized vs XSS-safe;
cache-hit vs cache-miss) left symmetric on purpose.

dashboard.md (70 more hits) is the next docs/web-ui/ batch, not part of
this one -- big enough to deserve its own PR.

docs/web-ui/terminal-rendering.md

@@ -4,8 +4,8 @@ The per-agent web UI's live pane renders one row per `TermMsg`
 (`hive-agent/src/term_msg.rs`): `{icon?, level: debug|info|warn|error,
 summary, body?, body_format?: markdown|diff, coalesce_key?}`. Classification
 (icon, summary text, whether a tool call gets an expandable body) happens
-server-side, once — `term_msg.rs` + `stream_enrich.rs` — and is served
-identically by both `GET /api/events/history` and `GET /api/events/stream`
+server-side, once — `term_msg.rs` + `stream_enrich.rs` — and both
+`GET /api/events/history` and `GET /api/events/stream` serve it identically
 as `TermEnvelope { ts, seq?, msgs: TermMsg[] }` frames
 (`hive-agent/src/web_ui/stream.rs`).