docs: changelogs and phase restructure for the models refactor (phase 8)

Breaking-change entries with migration guides: pi-ai (compat
entrypoint, Provider -> ProviderId, api module moves, new Models/
provider/auth API), agent (required AgentHarnessOptions.models,
compaction signatures, structural StreamFn), coding-agent (extension
author note: runtime unaffected via loader compat alias).

models.md: Phase 7 closed out (import switch landed with phase 5; the
gated bullets move to a new Phase 9 ModelManager-migration outline:
per-session Models, AgentSession streaming, AuthStorage replacement,
login UI on OAuthAuth, cloudflare cleanup, internal compat removal,
compat deletion).

packages/coding-agent/CHANGELOG.md

@@ -2,6 +2,10 @@
 
 ## [Unreleased]
 
+### Changed
+
+- pi-ai's old global API (`stream`/`complete`/`completeSimple`, `getModel`/`getModels`/`getProviders`, `registerApiProvider`, `getEnvApiKey`, ...) moved off the `@earendil-works/pi-ai` root entrypoint to `@earendil-works/pi-ai/compat`. Extensions are not affected at runtime: the extension loader resolves the pi-ai root to the compat entrypoint (a strict superset), so existing extensions keep working unchanged. Extension sources that typecheck against pi-ai's published types should switch those imports to `@earendil-works/pi-ai/compat` (or migrate to the new `createModels()`/provider-factory API). The compat entrypoint and the loader alias will be removed in a future release with a migration guide.
+
 ### Added
 
 - Added an experimental first-time setup flow behind `PI_EXPERIMENTAL=1` that asks for a dark/light theme choice (preselecting the detected appearance) and opt-in analytics data sharing on first launch with the default agent directory; opting in stores a `trackingId` in `settings.json`.