fix: verify and fix last-msg and pr-url activity field names

[GreyC]

Problem

last-msg and pr-url parse activity fields that were never verified against the real API:

• last-msg extracts text via lastMsg.message?.text → lastMsg.text → lastMsg.message (guessed)
• pr-url looks for a.completedSession?.pullRequestUrl (guessed)

These may silently return nothing on real sessions.

Fix

Run both commands against a real completed session with --json, inspect the raw activity structure, and update field paths to match actual API response.

Acceptance Criteria

• jules_cli last-msg <id> --json returns Jules's message text correctly
• jules_cli pr-url <id> returns the PR URL correctly