docs(changelog): em dash sweep — bring pre-session entries in line with style

The May 2026 mid-month catch-up commit (f68e7b1) added 12 new entries
written in period-separated style per Tori's writing-style preference.
The pre-session entries used em dashes liberally, creating two visible
voice styles within the same section.

Sweep replaces 141 em dashes across the whole file using two patterns:
- "**Title** — description" -> "**Title**. description" (dominant
  bold-lead pattern, 117 instances)
- mid-sentence " — " -> ", " (24 instances)

Title-internal em dashes (e.g. "MCP server — action tools") become
commas; mildly less readable but consistent. Optional polish later if
specific lines feel off.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>