Consolidate the duplicated helpers across Infrastructure projects #754

Several small helpers exist as verbatim copies across Infrastructure, and one copy has already drifted into a bug (the culture-sensitive timestamp parse, issue #723). Backlog.SharedKernel holds only Results, Handlers and Ai, so there is no natural home yet. Found in the 2026-09-27 repository review.

Context

Verified identical bodies:

• TrimForMessage three times: AzureFoundryChatClient.cs line 261, AzureFoundryCostClient.cs line 256, AzureFoundryEmbeddingsClient.cs line 125 (all under src/Infrastructure/Backlog.Infrastructure.AzureFoundry).
• Rfc3339 three times: GitHubActivityBaselineClient.cs line 166, GitHubActivityClient.cs line 1037, ClaudeUsageClient.cs line 366.
• Timestamp(JsonElement, string) four times (collapsed by Parse GitHub timestamps with the invariant culture in GitHubClient #723 if that lands first).
• PullRoute, Retired and WatermarkAfter three times each across TaskSyncClient/TaskSyncSession, Sessions/SessionSync* and Annotations/AnnotationSync* in src/Infrastructure/Backlog.Infrastructure.Sync, differing only by route constant or timestamp property.
• Unavailable/Expired exception factories three times across the Cosmos replicas in src/Infrastructure/Backlog.Infrastructure.Cosmos.
• OpenAsync plus PRAGMA journal_mode = WAL plus CREATE TABLE IF NOT EXISTS boilerplate three times: SqliteTaskRepository.cs line 268, Inbox/SqliteInboxRepository.cs line 287, Roadmap/SqliteRoadmapPlanRepository.cs line 157, and each runs the DDL on every open.

1. Add Backlog.SharedKernel/Text (or a per-project Internal static where the helper is truly project-local) for TrimForMessage, Rfc3339 and the timestamp parse.
2. Make the sync replica's route/retired/watermark trio one generic ReplicaRoutes<T> with a selector.
3. Extract a SqliteSchema.EnsureAsync(connection, ddl) that runs once per process per path, and one OpenAsync that sets WAL.
4. Add a test per extracted helper so behaviour is pinned before the copies are deleted.

Route through delivery:flow-code per CLAUDE.md.