docs(core): add v0.23.0 changelog entry

[phernandez]

Adds the complete v0.23.0 changelog section — the release-runbook prerequisite (just release pre-flight checks for the ## vX.Y.Z heading on main).

Drafted from the comprehensive change review in #1243 (141 PRs, v0.22.1 → main): intro
narrative, all ten flagged breaking changes, features led by the semantic-search and
concurrency-overhaul stories, bug fixes, and precedented maintenance entries. The
## Unreleased #997 entries are folded into the v0.23.0 section (reworded as the shipped
#1070 bm hook feature); an empty Unreleased heading remains for post-release work.

Release-day TODO: replace the 2026-08-XX date placeholder with the actual date.

Deliberate exclusions (triage closures, guard-level fixes subsumed by the
generation-versioning story, CI/deps/docs-only items, the 28-key config table → docs
reference) are recorded in #1243's discussion for review.

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https://claude.ai/code/session_01CPdSXDbYyhyZ1TwgFnpEv8

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[chatgpt-codex-connector]

Replace the release-date placeholder before landing

When this changelog entry lands on main as the release prerequisite, just release v0.23.0 accepts this heading because its preflight only checks for the version prefix, while scripts/update_versions.py does not replace the date. The release can therefore be published with the literal 2026-08-XX; use the intended release date now or add an enforced replacement step before treating the entry as release-ready.

AGENTS.md reference: AGENTS.md:L357-L359

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