feat: add /context slash command with a centered context modal

Typing /context in the composer opens a centered dialog with the current
composition (provider-anchored headline, stacked bar, legend) and the
last-10-turn trend chart — the same pushed contextTimeline projection the
tab renders, no RPC, no polling.

The command is the plugin's own '/' trigger source, fully client-side:
no host descriptor, no session log records, nothing model-visible. The
composer token is consumed only when the modal closes (deferred
slash/input-consume-token with span / bare-token guards, fail-soft if the
draft changed meanwhile). Esc, backdrop click, and the close button all
close it and restore composer focus.

Also extracts the provider-anchored headline derivation shared by the tab
and the modal (headline.ts).

src/client/command.ts

@@ -0,0 +1,55 @@
+/**
+ * `/context` — a client-owned slash command that opens the context modal
+ * (current composition + recent trend) in the center of the page.
+ *
+ * Implemented as the plugin's own '/' trigger source instead of a host
+ * command: nothing is dispatched to the host, no session log records are
+ * written, and nothing becomes model-visible — the invocation never enters
+ * the message history. Both paths answer `'handled'` and open the modal,
+ * leaving the `/context` token in the composer while it is open; the modal's
+ * close path consumes the token then (see modalStore.ts).
+ */
+
+import { modalStoreOf, setPendingConsume } from './modalStore'
+import type { ClientCtx, InputTriggersFace } from './services'
+import type { ViewKit } from './viewkit'
+
+const COMMAND = 'context'
+const LINE = '/' + COMMAND
+
+export function registerContextCommand(ctx: ClientCtx, kit: ViewKit): void {
+  ctx.effect(() => {
+    // Soft dependency (ctx.get, not the inject list): a harness without the
+    // input-trigger service keeps the tab working and only the command
+    // absent.
+    const inputTriggers = ctx.get('inputTriggers') as InputTriggersFace | undefined
+    if (inputTriggers === undefined) return () => {}
+
+    return inputTriggers.registerSource({
+      trigger: '/',
+      name: COMMAND,
+      order: 1,
+      candidates: (_session, req) => {
+        if (req.position !== 'leading') return Promise.resolve([])
+        const query = req.query.trim().toLowerCase()
+        if (query !== '' && !COMMAND.startsWith(query)) return Promise.resolve([])
+        return Promise.resolve([{ name: COMMAND, description: kit.t('cmd.desc') }])
+      },
+      onPick: (pick) => {
+        // Open and remember the token span: the modal's close path consumes
+        // it (span CAS — a draft changed meanwhile is left untouched).
+        setPendingConsume(pick.session.sessionId, { kind: 'span', span: pick.span })
+        modalStoreOf(pick.session.sessionId).set(true)
+        return 'handled'
+      },
+      matchEnter: (session, line) => {
+        if (line !== LINE) return Promise.resolve(undefined)
+        // Same deferred consume, bare-token guard (the trimmed draft must
+        // still equal the command line when the modal closes).
+        setPendingConsume(session.sessionId, { kind: 'bare-token', token: LINE })
+        modalStoreOf(session.sessionId).set(true)
+        return Promise.resolve<'handled'>('handled')
+      },
+    })
+  }, 'dsh-context: /context command')
+}