docs(changelog): fix duplicate Unreleased — fold stale block into v0.11.0-rc.3 extended notes

The CHANGELOG had two `## [Unreleased]` headings — one active (queued
for the next release-please rollup) and a 1547-line stale block at
line 418 with rich manual narrative for campaigns that shipped in
v0.11.0-rc.3 and v0.10.0-rc.3 (System audit refactor, Living Wiki
cold-start reliability, Fresh-install LLM onboarding, Orchestrator
capacity detection). release-please's auto-generated bullets covered
the same campaigns but tersely — the rich prose was orphaned.

Rename the stale heading to `## [0.11.0-rc.3] — extended notes` and
reposition it directly under v0.11.0-rc.3's autogenerated section, so
the deeper narrative reads as the version's audit-trail commentary
rather than as a parallel unreleased section. No content removed; only
the heading and its position changed.

Result: one `## [Unreleased]` block (active), correct sectioning for
release-please's next rollup, full historical narrative preserved.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: jstuart0