feat(agents): custom context for Tier 2 + periodic system prompt re-injection

- SettingsTab: Custom Context textarea for Tier 2 project cards
- AgentSession passes systemPrompt for ALL projects (Tier 1 gets full
  generated prompt, Tier 2 gets custom context)
- Periodic re-injection: 1-hour timer checks if agent is idle, then
  auto-sends context refresh prompt with role/tools reminder
- AgentPane: autoPrompt prop consumed when session is done/error,
  resumes session with fresh system prompt

v2/src/lib/components/Agent/AgentPane.svelte

@@ -58,10 +58,14 @@
     agentSystemPrompt?: string;
     /** Extra env vars injected into agent process (e.g. BTMSG_AGENT_ID) */
     extraEnv?: Record<string, string>;
+    /** Auto-triggered prompt (e.g. periodic context refresh). Picked up when agent is idle. */
+    autoPrompt?: string;
+    /** Called when autoPrompt has been consumed */
+    onautopromptconsumed?: () => void;
     onExit?: () => void;
   }
 
-  let { sessionId, projectId, prompt: initialPrompt = '', cwd: initialCwd, profile: profileName, provider: providerId = 'claude', capabilities = DEFAULT_CAPABILITIES, useWorktrees = false, agentSystemPrompt, extraEnv, onExit }: Props = $props();
+  let { sessionId, projectId, prompt: initialPrompt = '', cwd: initialCwd, profile: profileName, provider: providerId = 'claude', capabilities = DEFAULT_CAPABILITIES, useWorktrees = false, agentSystemPrompt, extraEnv, autoPrompt, onautopromptconsumed, onExit }: Props = $props();
 
   let session = $derived(getAgentSession(sessionId));
   let inputPrompt = $state(initialPrompt);
@@ -145,6 +149,16 @@
   // NOTE: Do NOT stop agents in onDestroy — it fires on layout changes/remounts,
   // not just explicit close. Stop-on-close is handled by workspace teardown.
 
+  // Auto-prompt: pick up externally triggered prompts (e.g. periodic context refresh)
+  $effect(() => {
+    if (!autoPrompt || isRunning) return;
+    // Only trigger if session exists and is idle (done/error)
+    if (!session || (session.status !== 'done' && session.status !== 'error')) return;
+    const prompt = autoPrompt;
+    onautopromptconsumed?.();
+    startQuery(prompt, true); // resume session with context refresh
+  });
+
   let promptRef = $state<HTMLTextAreaElement | undefined>();
 
   async function startQuery(text: string, resume = false) {

v2/src/lib/components/Workspace/AgentSession.svelte

@@ -1,4 +1,5 @@
 <script lang="ts">
+  import { onDestroy } from 'svelte';
   import type { ProjectConfig, GroupAgentRole } from '../../types/groups';
   import { generateAgentPrompt } from '../../utils/agent-prompts';
   import { getActiveGroup } from '../../stores/workspace.svelte';
@@ -24,6 +25,9 @@
   import { SessionId, ProjectId } from '../../types/ids';
   import AgentPane from '../Agent/AgentPane.svelte';
 
+  /** How often to re-inject the system prompt (default 1 hour) */
+  const REINJECTION_INTERVAL_MS = 60 * 60 * 1000;
+
   interface Props {
     project: ProjectConfig;
     onsessionid?: (id: string) => void;
@@ -34,8 +38,9 @@
   let providerId = $derived(project.provider ?? getDefaultProviderId());
   let providerMeta = $derived(getProvider(providerId));
   let group = $derived(getActiveGroup());
+  // Build system prompt: full agent prompt for Tier 1, custom context for Tier 2
   let agentPrompt = $derived.by(() => {
-    if (!project.isAgent || !project.agentRole || !group) return undefined;
+    if (project.isAgent && project.agentRole && group) {
       return generateAgentPrompt({
         role: project.agentRole as GroupAgentRole,
         agentId: project.id,
@@ -43,6 +48,9 @@
         group,
         customPrompt: project.systemPrompt,
       });
+    }
+    // Tier 2: pass custom context directly (if set)
+    return project.systemPrompt || undefined;
   });
 
   // Inject BTMSG_AGENT_ID for agent projects so they can use btmsg/bttask CLIs
@@ -51,6 +59,36 @@
     return { BTMSG_AGENT_ID: project.id };
   });
 
+  // Periodic context re-injection timer
+  let lastPromptTime = $state(Date.now());
+  let contextRefreshPrompt = $state<string | undefined>(undefined);
+  let reinjectionTimer: ReturnType<typeof setInterval> | null = null;
+
+  function startReinjectionTimer() {
+    if (reinjectionTimer) clearInterval(reinjectionTimer);
+    lastPromptTime = Date.now();
+    reinjectionTimer = setInterval(() => {
+      const elapsed = Date.now() - lastPromptTime;
+      if (elapsed >= REINJECTION_INTERVAL_MS && !contextRefreshPrompt) {
+        const refreshMsg = project.isAgent
+          ? '[Context Refresh] Review your role and available tools above. Check your inbox with `btmsg inbox` and review the task board with `bttask board`.'
+          : '[Context Refresh] Review the instructions above and continue your work.';
+        contextRefreshPrompt = refreshMsg;
+      }
+    }, 60_000); // Check every minute
+  }
+
+  function handleAutoPromptConsumed() {
+    contextRefreshPrompt = undefined;
+    lastPromptTime = Date.now();
+  }
+
+  // Start timer and clean up
+  startReinjectionTimer();
+  onDestroy(() => {
+    if (reinjectionTimer) clearInterval(reinjectionTimer);
+  });
+
   let sessionId = $state(SessionId(crypto.randomUUID()));
   let lastState = $state<ProjectAgentState | null>(null);
   let loading = $state(true);
@@ -60,6 +98,8 @@
     sessionId = SessionId(crypto.randomUUID());
     hasRestoredHistory = false;
     lastState = null;
+    lastPromptTime = Date.now();
+    contextRefreshPrompt = undefined;
     registerSessionProject(sessionId, ProjectId(project.id), providerId);
     trackProject(ProjectId(project.id), sessionId);
     onsessionid?.(sessionId);
@@ -153,6 +193,8 @@
       useWorktrees={project.useWorktrees ?? false}
       agentSystemPrompt={agentPrompt}
       extraEnv={agentEnv}
+      autoPrompt={contextRefreshPrompt}
+      onautopromptconsumed={handleAutoPromptConsumed}
       onExit={handleNewSession}
     />
   {/if}

v2/src/lib/components/Workspace/SettingsTab.svelte

@@ -941,6 +941,20 @@
               </div>
             </div>
 
+            <div class="card-field">
+              <span class="card-field-label">
+                <svg width="12" height="12" viewBox="0 0 24 24" fill="none" stroke="currentColor" stroke-width="2" stroke-linecap="round" stroke-linejoin="round"><path d="M14 2H6a2 2 0 0 0-2 2v16a2 2 0 0 0 2 2h12a2 2 0 0 0 2-2V8z"/><polyline points="14 2 14 8 20 8"/><line x1="16" y1="13" x2="8" y2="13"/><line x1="16" y1="17" x2="8" y2="17"/></svg>
+                Custom Context
+              </span>
+              <textarea
+                class="agent-prompt-input"
+                value={project.systemPrompt ?? ''}
+                placeholder="Additional instructions injected into this project's agent session"
+                rows="3"
+                onchange={e => updateProject(activeGroupId, project.id, { systemPrompt: (e.target as HTMLTextAreaElement).value || undefined })}
+              ></textarea>
+            </div>
+
             <div class="card-footer">
               <button class="btn-remove" onclick={() => removeProject(activeGroupId, project.id)}>
                 <svg width="12" height="12" viewBox="0 0 24 24" fill="none" stroke="currentColor" stroke-width="2" stroke-linecap="round" stroke-linejoin="round"><path d="M3 6h18"/><path d="M8 6V4h8v2"/><path d="M19 6v14a2 2 0 0 1-2 2H7a2 2 0 0 1-2-2V6"/></svg>