feat: Propagate task summaries and input context to execution agents

Execution agents were spawning blind — no input files, no knowledge of
what predecessor tasks accomplished. This adds three capabilities:

1. summary column on tasks table — completeTask() reads the finishing
   agent's result.message and stores it on the task record
2. dispatchNext() gathers full initiative context (initiative, phase,
   sibling tasks, pages) and passes it as inputContext so agents get
   .cw/input/task.md, initiative.md, phase.md, and context directories
3. context/tasks/*.md files now include the summary field in frontmatter
   so dependent agents can see what prior agents accomplished

apps/server/agent/file-io.ts

@@ -247,6 +247,7 @@ export function writeInputFiles(options: WriteInputFilesOptions): void {
           type: t.type,
           priority: t.priority,
           status: t.status,
+          summary: t.summary,
         },
         t.description ?? '',
       );