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/task-management.md

@@ -855,6 +855,8 @@ Users can apply presets at task creation; manual model selection can override th
 
 When `autoSummarizeTitles` is enabled and a task has a long untitled description, Fusion can auto-generate a concise title. This applies to tasks created from the dashboard/API as well as tasks created by agents and tooling flows (`fn_task_create`, delegated tasks, and triage-created child tasks). GitHub tracking now waits for the `createTask`-level summarizer (explicit or auto-attached from settings) to settle before filing, then uses that resulting title and falls back to deterministic description-derived title generation only when summarization is unavailable.
 
+If a configured title summarizer model is stale after a pi upgrade, Fusion logs a warning naming that provider/model and retries once with automatic model resolution before falling back to deterministic title generation. Genuine AI-service failures are not masked by this retry.
+
 ## Screenshots
 
 ### Board/task cards + quick entry