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/agent/CHANGELOG.md

@@ -2,6 +2,12 @@
 
 ## [Unreleased]
 
+### Breaking Changes
+
+- `AgentHarnessOptions.models` is required: the harness streams turns, compaction, and branch summarization through the provided `Models` instance (`models.streamSimple()`/`completeSimple()`) instead of the pi-ai global stream functions. Build one with `createModels()` + provider factories (or `builtinModels()` from `@earendil-works/pi-ai/providers/all`); tests use `fauxProvider()`. `getApiKeyAndHeaders` still wins per field but is no longer required — without it, requests resolve auth through the providers.
+- `compact()`, `generateSummary()`, and `generateBranchSummary()` take a `Models` parameter; the explicit `apiKey` is now optional.
+- `StreamFn` is defined structurally (`(model, context, options?) => AssistantMessageEventStream | Promise<...>`); `Models.streamSimple` satisfies it.
+
 ## [0.79.1] - 2026-06-09
 
 ## [0.79.0] - 2026-06-08