fix(cron): clear scheduled jobs when sessions change

[outlier27-cell]

Problem

/cron documents scheduled prompts as session-scoped, but a second Pi session_start retained the preceding session's in-memory jobs. A due old-session prompt could therefore be delivered after /new or /resume.

Value

Scheduled prompts now remain owned by the session where the user created them, preventing cross-session follow-ups.

Approach

• Reset cron-owned jobs, the ID sequence, and the polling timer before accepting a new session context.
• Add a lifecycle regression covering a second session_start, absence of the old job, and normal scheduling in the new session.

Validation

• node --test --experimental-strip-types tests/extensions/cron/index.test.ts tests/extensions/cron/schedule.test.ts (23 passed)
• bun run check (passed: config/docs/disciplines, Web build, format, lint, and typecheck)
• bun run test was started, but unrelated Git/process, renderer, and workflow tests failed under the Windows parallel runner and the run later stalled. The cron suite itself passed in that run. Exact-head GitHub CI is required before merge.

Impact

No persistent state, provider behavior, or model-facing surface changes. Existing jobs are intentionally cleared whenever Pi begins a new session lifecycle.

Closes #736

[outlier27-cell]

@tt-a1i 请审核此 PR。#736 的跨 Session 定时任务泄漏已在 c686292 修复；exact-head GitHub CI（Node、Windows、UI、Web E2E）均通过。当前仅等待审核。