daily-focus-board: explicit op results + date-scoped default state file

Resolves two more Copilot review comments on board-core.mjs:
- Mutations no longer report false success: every op returns an explicit error
  for unknown task ids, wrong task kinds, and empty/no-op inputs, and mutate
  propagates it (so set_status on a bad id fails instead of claiming { ok: true }).
- The default state filename is now date-scoped (focus-board-<YYYY-MM-DD>.json),
  so reopening the board tomorrow starts a fresh day instead of silently reusing
  today's tasks/progress. Explicit stateFile paths are used as-is; input doc updated.

Headless test 39/39 (adds op-error + date-scope cases).

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
Copilot-Session: cb356aa8-0af2-48f3-b3c6-8086c69d5308

extensions/daily-focus-board/extension.mjs

@@ -36,7 +36,7 @@ const session = await joinSession({
                 properties: {
                     stateFile: {
                         type: "string",
-                        description: "Absolute path to the board's JSON state file. Seeded if it doesn't exist yet. Defaults to focus-board-state.json in the current working directory.",
+                        description: "Absolute path to the board's JSON state file. Seeded if it doesn't exist yet. Defaults to a date-scoped focus-board-<YYYY-MM-DD>.json in the current working directory, so each day starts a fresh board; pass an explicit path to keep a single ongoing file.",
                     },
                     seed: {
                         type: "object",