[Agent Loop] Registry-backed tool resolution + RBAC + audit + agent docs #852

Part of #612. Depends on #615, #618, #622.

Swap the agent loop's v1 caller-supplied opts.toolHandlers dispatch for registry-backed tool resolution — the half of #612 that makes toolMode: 'auto' the compressed-stack story (auto-discovered, RBAC-filtered, audited tools) rather than hand-wired handlers.

Scope / acceptance

• Resolve tool calls against scope.resources — an @export-ed Resource's get_*/search_*/create_*/… verbs are callable as tools ([MCP] Application profile: tool generation over Resources registry #618).
• Resolve MCP-registered tools via the shared registry ([MCP] Tool registry + class-level introspection + RBAC-aware tools/list filtering #615) and custom mcpTools opt-ins ([MCP] Custom Resource opt-in via static mcpTools declaration #622).
• Tool calls run with the caller's identity — RBAC enforced via the registry's filtering, not the caller's handler.
• Tool calls audited in the transaction log alongside data writes (inherited from Resource dispatch).
• Docs: building an agent with auto mode, registering a Resource as a tool, error/budget/abort/conversation semantics.

Seam already in place (PR #848)

The loop's missing-handler split — DECLARED-but-no-handler → hard ClientError(400); UNDECLARED → recoverable "unknown tool" — maps directly onto the registry's resolvable-but-misconfigured vs unknown cases. The swap is a lookup replacement in runSingleToolCall, not a loop rewrite.

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