feat(report): drilldown reverse + offset options — let users reach the newest messages

[Tyan66666]

Closes #93

Problem

When agent opens the acp_status drilldown (scope:"uncompressed" + view:"messages", sort:"time"), the list always starts at the oldest message. slice(0, limit) only ever shows the head, so once the conversation is longer than limit, the newest messages are simply unreachable — there is no way to flip the order (reverse), page forward (offset), or sort newest-first. The same head-only limitation applies to sort:"size" / sort:"tool" and to scope:"compressed".

Why it matters differs by host:

• Hosts that inject <acp> ref tags into the message stream (e.g. billion-context-pi) see every message inline — the drilldown is a nice-to-have there.
• Hosts without tags (billion-context-dsh: no tags, seq-is-the-ref) rely on the drilldown as their only way to locate messages — for them the head-only behavior is a real gap (tracking: /acp status 查看消息记录总是只显示最早的一段，最新消息看不到（等上游 reverse/offset 修复后关闭） Tyan66666/billion-context-dsh#46).

Fix

Two new optional options on StatusReportOptions (backwards-compatible, no behavior change when omitted):

• reverse?: boolean — after sorting, flip the order. sort:"time" + reverse:true gives newest-first, so the tail of a long conversation becomes reachable.
• offset?: number — pagination. Rows are windowed with slice(offset, offset + limit).

Applied in renderMessageDrilldown and renderCompressedDrilldown (both honor reverse then window with offset). Footer text: offset 0 keeps the existing N of M shown.; offset > 0 prints Showing A–B of M.; an offset past the end prints Offset N past end (M total). instead of a silent empty page. The overview Tip now advertises sort:"time" + reverse:true for newest-first, and README's drilldown options table + example are updated.

Usage:

buildStatusReport(state, messages, countTokens, {
  scope: "uncompressed",
  view: "messages",
  sort: "time",
  reverse: true,   // newest message first — reach the tail
  limit: 30,
});
// pagination: offset: 30 → the next page

Tests

7 new cases in tests/report-drilldown.test.ts: status-quo pin (oldest-first head-only), reverse:time reaches the newest messages, offset pagination in both directions, offset past end, sort:"size" reverse (smallest-first), tool filter + reverse, and scope:"compressed" reverse + pagination. Full suite: 444 pass, typecheck + build clean.