[proxy] Capture sub-agent ids from LLM request headers

A coding agent that spawns helpers stamps the spawned agent's id on every
request it makes, and the spawning agent's id when that helper is nested.
The parser read the session header and ignored both, so parallel agents
inside one session all attributed to the session alone and there was no way
to see which one spent the tokens.

Emit them as metadata alongside the session id. They are opaque grouping
identifiers rather than content, so they are stamped regardless of the
prompt-collection toggle. Persisting them as queryable access-log columns
is a schema change and is deliberately not part of this commit.

proxy/internal/middleware/keys.go

@@ -22,6 +22,15 @@ const (
 	// body. Empty for clients that don't send one.
 	KeyLLMSessionID = "llm.session_id"
 
+	// Sub-agent attribution (emitted by llm_request_parser from the
+	// client's request headers). A coding agent that spawns helpers
+	// stamps the spawned agent's id, and the spawning agent's id when
+	// the helper is itself nested, so cost within one session can be
+	// split across the agents that ran in parallel. These identify an
+	// agent, not a person or a device: never treat them as a user id.
+	KeyLLMAgentID       = "llm.agent_id"
+	KeyLLMParentAgentID = "llm.parent_agent_id"
+
 	// LLM response-side metadata (emitted by llm_response_parser).
 	//nolint:gosec // metadata key name, not a credential
 	KeyLLMInputTokens = "llm.input_tokens"