feat: Add Details tab to agent right-panel with metadata, input files, and prompt sections

Adds an Output/Details tab bar to the agents page right-panel. The Details
tab renders AgentDetailsPanel, which surfaces agent metadata (status, mode,
provider, initiative link, task name, exit code), input files with a
file-picker UI, and the effective prompt text — all streamed via the new
getAgent/getAgentInputFiles/getAgentPrompt tRPC procedures.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

docs/frontend.md

@@ -44,6 +44,7 @@ Use `mapEntityStatus(rawStatus)` from `StatusDot.tsx` to convert raw entity stat
 |-------|-----------|---------|
 | `/` | `routes/index.tsx` | Dashboard / initiative list |
 | `/initiatives/$id` | `routes/initiatives/$initiativeId.tsx` | Initiative detail (tabbed) |
+| `/agents` | `routes/agents.tsx` | Agent list with Output / Details tab panel |
 | `/settings` | `routes/settings/index.tsx` | Settings page |
 
 ## Initiative Detail Tabs
@@ -54,7 +55,7 @@ The initiative detail page has three tabs managed via local state (not URL param
 2. **Execution Tab** — Pipeline visualization, phase management, task dispatch
 3. **Review Tab** — Pending proposals from agents
 
-## Component Inventory (73 components)
+## Component Inventory (74 components)
 
 ### Core Components (`src/components/`)
 | Component | Purpose |
@@ -66,6 +67,7 @@ The initiative detail page has three tabs managed via local state (not URL param
 | `StatusBadge` | Colored badge using status tokens |
 | `TaskRow` | Task list item with status, priority, category |
 | `QuestionForm` | Agent question form with options |
+| `AgentDetailsPanel` | Details tab for agent right-panel: metadata, input files, effective prompt |
 | `InboxDetailPanel` | Agent message detail + response form |
 | `ProjectPicker` | Checkbox list for project selection |
 | `RegisterProjectDialog` | Dialog to register new git project |