FN-6218: restore pi upgrade compatibility

Restore Pi-upgraded workspaces to keep Fusion resources accessible and recover title generation when configured models go stale.

- Mark read-only Fusion/Pi provider settings views as project-trusted so extension loading keeps working after Pi upgrades.
- Retry task title summarization with automatic model resolution when the configured provider model is missing from the Pi registry.
- Cover provider settings trust behavior and stale summarizer fallback paths with regression tests.
- Document the read-only settings trust contract and add a patch changeset.

Files changed:
 .changeset/fn-6218-pi-upgrade-regressions.md       |   5 +
 docs/settings-reference.md                         |   2 +
 docs/task-management.md                            |   2 +
 .../commands/__tests__/provider-settings.test.ts   |  16 ++++
 packages/cli/src/commands/provider-settings.ts     |   5 +
 packages/core/src/__tests__/ai-summarize.test.ts   | 102 +++++++++++++++++++++
 packages/core/src/ai-summarize.ts                  |  85 ++++++++++++-----
 .../src/__tests__/pi-create-fn-agent.test.ts       |  32 +++++++
 packages/engine/src/pi.ts                          |   7 +-
 9 files changed, 233 insertions(+), 23 deletions(-)

Fusion-Task-Id: FN-6218

Fusion-Task-Lineage: 3d6aca32-a56a-43a0-a57b-8e6abc839ce9

docs/settings-reference.md

@@ -837,6 +837,8 @@ Project-scoped model lane used for task title auto-summarization, GitHub trackin
 5. Global `defaultProvider` + `defaultModelId`
 6. Automatic provider/model resolution
 
+If the configured title summarizer provider/model is stale and no longer exists in the pi model registry, title generation logs a warning with the stale id and retries once with automatic provider/model resolution. Other AI failures (auth, empty output, unavailable engine) still fail normally.
+
 > **Note:** Runtime fallback precedence logic is implemented in engine and dashboard routes. The hierarchies above reflect current runtime behavior.
 
 ---