refactor: extract session autosave coordinator

[teamleaderleo]

Problem

AppDelegate owned the session autosave timer, typing quiet-period retry, in-flight state, process-scan generation guards, fingerprint deduplication, and persisted autosave state.

Change

Extract that lifecycle policy into SessionAutosaveCoordinator. It owns timer/tick state, generation checks, fingerprint skip state, and a cancellable ContinuousClock retry task. AppDelegate now supplies only fingerprint/save closures and forwards start, stop, typing activity, and generation checks. Snapshot construction, persistence, and lifecycle observers remain in their existing owners.

This PR is stacked on #72.

Validation

• swiftc -D DEBUG -parse Sources/SessionAutosaveCoordinator.swift Sources/AppDelegate.swift
• git diff --check

A full xcodebuild was attempted but blocked before compilation because this fork worktree is missing vendor/bonsplit/Package.swift (uninitialized dependency).

[teamleaderleo]

Consolidated into the coherent upstream slices #78 (UI-test infrastructure), #79 (session lifecycle), #80 (window ownership), and #82 (terminal diagnostics). The original branch remains available for history.