feat(agent): stable child-scoped provider identity for RLM subagents, side questions, and auxiliary requests

[rynfar]

Part of #22 (Meridian reliability umbrella). P0 — the biggest single lever on prompt-cache churn.

Problem

Anthropic-protocol providers have exactly one session-identity channel: metadata.user_id, set only in packages/ai/src/providers/anthropic.ts (buildParams). The Meridian extension fills it from ctx.sessionManager.getSessionId() in before_provider_request. That identity is wrong or missing in four cases:

1. Inline RLM children stamp the parent's key. _createInlineRlmSubagentRuntime (packages/coding-agent/src/core/agent-session.ts) constructs the child Agent with onPayload: this.agent.onPayload — the root closure from packages/coding-agent/src/core/sdk.ts bound to the root extensionRunnerRef and therefore the root session manager. Every inline child request carries the parent's metadata.user_id. Parent + N children interleaving on one proxy session key means the incoming history never matches what the key last saw, forcing a full fresh-session replay (guaranteed prompt-cache miss, and Meridian's replay drops assistant tool_use blocks — Fresh replay drops every assistant tool_use block, so a thinking-plus-tool-call turn becomes an empty turn rynfar/meridian#888 — losing child task context). Daemon-mode children get their own runner and behave correctly; behavior diverging by mode is itself a bug.
2. Identity mutates mid-conversation. SessionManager.sessionId changes on fork/branch/load (session-manager.ts newSession, createBranchedSession), so a continuing conversation shows up at the proxy under a new key with old history → lineage=new divergence mid-session (observed in telemetry: lineage=new session=new msgCount=48).
3. Side questions reuse the parent's key with a forked message array (packages/coding-agent/src/core/side-question.ts reuses parent.onPayload and parent.sessionId) → same key, divergent prefix → churn.
4. Compaction, branch summarization, and refinement are unkeyed. compaction/compaction.ts, compaction/branch-summarization.ts, and refinement/refinement.ts call completeSimple directly with no onPayload/metadata → they reach the proxy with no session identity at all.

Proposed shape

• Add a first-class, immutable per-AgentSession identity (survives fork/branch/load; distinct per RLM child and per side question, e.g. <rootId>, <rootId>/child:<childId>, <rootId>/side:<n>), exposed on AgentOptions/AgentLoopConfig and to extension contexts, rather than deriving identity from the mutable SessionManager.sessionId.
• Make inline RLM children run their payload hooks through the child's extension runner (pass a child-scoped extensionRunnerRef instead of copying the parent's onPayload/onResponse), converging inline and daemon behavior.
• Thread identity/onPayload through the four unkeyed completeSimple call sites.
• Keep the extension contract additive: metadata.user_id remains a JSON envelope; a parent linkage field (e.g. parent_session_id) enables proxy-side parent→child cancellation later (Meridian reliability: make concurrent RLM subagents production-safe over Claude Max #22 sequencing).

Acceptance

• With the Meridian extension active: a parent with 3 concurrent inline RLM children produces 4 distinct stable metadata.user_id values, unchanged across a branch/fork operation, across every request each agent makes (including compaction and side questions carrying their owner's identity or a derived sub-identity — never a sibling's).
• Focused tests at the seams: inline child runner scoping, identity stability across createBranchedSession, keyed completeSimple call sites.