Platform toolset routing and hook telemetry attribution

This patch release moves assistant platform toolsets onto the /platform/mcp/ path prefix, makes hook telemetry filterable in Datadog by org, project, source, and event, and fixes the Codex Stop hook in Cowork.

Bug fixes

• Platform toolsets at /platform/mcp/{slug} #2823  - Assistant platform toolsets are now served from /platform/mcp/{slug} instead of /x/platform-mcp/{slug}, in line with the /mcp prefix for MCP servers. (Author: @danielkov )
• Dedupe chat asset writes #2839  - Dedupes chat asset writes and idempotently uploads to prevent GCS 429s. (Author: @bflad )
• Structured hook telemetry attributes #2838  - Makes hook routes (Claude, Cursor, Codex, OTEL Logs, OTEL Metrics) filterable in Datadog by gram.org.id, gram.project.id, gram.hook.source, and gram.hook.event. Replaces nested value payloads with top-level slog attrs and logs on every early-return path so silent 401s and missing-session-id branches are visible when debugging hook setup. (Author: @bradcypert )
• Codex Stop hook in Cowork #2842  - Gets the Stop hook working in Cowork again. (Author: @chase-crumbaugh )