docs: changelog and blog post for v0.9.2 (#3765)

* docs: changelog and blog post for v0.9.2

* docs: rework the 0.9.2 blog post and flag the last_refreshed_at break

- drop the 'largest reliability release' framing and the movie-style section
  titles; plain, specific headings for a point release
- Recall: name the actual field and shape, one bullet per change
- replace the correctness section with Mental Models, covering the delta
  markdown corruption, retracted-fact unsay, the last_refreshed_at semantics,
  refresh pacing/dedupe and the new refresh outcome
- retitle 'Built for Many Banks' -> 'Running Many Banks'
- add 'Retain on Large Documents' for #3763
- promote last_refreshed_at to Breaking Changes in the changelog, where its
  commit already labelled it

* docs: keep last_refreshed_at a bug fix, not a breaking change

* docs: concrete opener and specific section titles for the 0.9.2 post

- open on the actual defects (welded table, 6000 refreshes/day) instead of
  framing the release as four things the engine decided by itself
- every section title now names what changed rather than the area it touched
- drop the 15-of-16-constructs statistic from the markdown-corruption bullet

* docs: lead the 0.9.2 post with features, not bug fixes

Rebuild the post around what 0.9.2 adds: knowledge-base MCP tools, the
caller-supplied recall window, the mental-model refresh floor and typed refresh
outcome, the Copilot provider and asymmetric embedding prefixes, and the two new
operator ceilings (retain memory budget, vector-index row threshold).

The mental-model corruption, retracted-fact and last_refreshed_at narratives are
gone; fixes now collapse into one closing line pointing at the changelog, which
carries all 44 of them.

* docs: regenerate the 0.9.2 changelog on the rebased main

Picks up #3763, #3764, #3770 and #3783, which landed after the first run.
Re-adds eight entries the summarizer dropped again: the Copilot provider and
knowledge-base MCP tools (both headlined in the blog post), bank-list
pagination, worker tracing, last_refreshed_at, the maintenance DDL deadlock,
Vertex AI multi-region, and the Windows daemon windows.

Also notes the sub-batch split streaming in the retain section of the post.

* docs: add the 0.9.2 release infographic to the blog post

* docs: sync the skill openapi.json to 0.9.2

release.sh regenerates the docs skill before bumping the OpenAPI version, so
the skill's copy lags the release commit by one version.