feat(observability): group agent storage spans

[Ankcorn]

Why

cloudflare#1860 makes inference and tool work visible, but SDK-managed chat flows still emit dozens of Durable Object spans beside the useful invoke_agent path.

This groups storage work into semantic phases so raw DO spans remain available without burying model and tool activity.

Trace shape

agent_initialization
└── initialize_agent_storage
    └── DO schema/migration spans...

think_start
├── initialize_think_session
├── hydrate_think_session
├── initialize_think_chat
├── agent_start
│   ├── restore_agent_state
│   ├── restore_mcp_connections
│   ├── recover_agent_work
│   └── run_user_on_start
├── reconcile_think_schedules
└── recover_think_durable_work

chat_interaction
├── clear_previous_chat_state
├── persist_chat_request_context
├── load_chat_history
├── persist_incoming_messages
├── schedule_agent_alarm
└── chat_turn
    ├── initialize_fiber
    ├── persist_fiber_snapshot
    ├── prepare_agent
    │   └── tools/skills/session/prompt/config DO spans...
    ├── invoke_agent
    │   └── chat
    │       └── execute_tool
    ├── persist_chat_result
    ├── finalize_fiber
    └── schedule_agent_alarm

prepare_agent closes before invoke_agent starts, keeping them as siblings. No known SDK SQLite spans remain as direct siblings of invoke_agent.

Durable submissions similarly group acceptance/execution around a nested chat_turn.

Attributes

Every SDK span carries agent identity plus:

cloudflare.agents.operation.name
cloudflare.agents.storage.grouped = true
cloudflare.agents.storage.system = "durable_object"
cloudflare.agents.storage.phase

Chat phases also carry request ID, trigger, admission, generation, channel and continuation where available. Initialization/startup spans add schema, hydration and recovery metadata. Conversation content, tool payloads and error messages are not recorded.

A phase typically groups 3–15 DO operations; full schema migration groups roughly 45, while streaming and recovery are unbounded.

One runtime-level KV read can remain between initialization and startup: PartyServer privately hydrates its legacy __ps_name before invoking onStart, outside Agent lifecycle hooks.

Stacked on cloudflare#1860. This fork PR targets an exact mirror of that PR head because this account cannot open a PR directly against Cloudflare's branch.