Wired the generic discovery methods on AgentSession into the tool
factory session, resolved the effective discovery mode (tools.discovery
Mode wins; mcp.discoveryMode as back-compat alias for "mcp-only"),
and threaded the resulting flag into rebuildSystemPrompt so the prompt
template fires the discovery hint for both legacy and new modes.
Added the load-bearing filter in createAgentSession: when the
effective mode is "all", drop any built-in tool whose loadMode is
"discoverable" from the initial tool set unless it is essential per
computeEssentialBuiltinNames, was explicitly listed via
options.toolNames, or was restored from persistence. The model finds
hidden tools via search_tool_bm25 and activates them on demand.
Built-in activation persistence is intentionally limited to the
existing MCP persistence store for this PR; full discovered-tool
persistence is a follow-up.
