docs(changelog): audit [Unreleased] — tighten for skimmability

Calibrates the main [Unreleased] against v2.7.0's detail level. Collapses
four scattered ### Added / two ### Changed / two ### Fixed headers into
one of each, shrinks the Summary from 8 paragraphs to 1 (with a migration
guide link), trims per-bullet implementation detail, and merges the split
Plan C narrative (IoChain dst_tier, external_potential, status/provenance,
strict-stdlib cull, HuggingFace revert) so a reader can skim the whole
release in under a minute.

Tracker [Unreleased] groups the four transient-.ops read-race entries
under a single subheading and removes hard-wrapping so each bullet is
one line.

Every WI-ID, the INV-miniz / INV-maluk references, the
awaits_bakeoff_validation tag, and the schema 0.2.2 → 0.2.3 bump are
preserved; nothing removed from the record, only condensed. CHANGELOG.md
drops 122 lines net; tracker drops 22.

Signed-off-by: jgstern-agent <josh-agent@iterabloom.com>

Author: jgstern-agent

.ci/affected-tests.txt

@@ -1,9 +1,9 @@
 # Test selection manifest
-# Generated by smart-test at 2026-04-24T13:13:15-04:00
+# Generated by smart-test at 2026-04-24T13:55:30-04:00
 # Mode: targeted
 # Baseline: db497c028ce764d25808f87a205e8a01c315766e
 # Reason: no Python source files changed
-# Changed files: 4
+# Changed files: 5
 # Changed source files: 0
 # Selected tests: 0
 #